| Message ID | cover.1713755580.git.baolin.wang@linux.alibaba.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | add mTHP support for anonymous share pages | expand |
On 22/04/2024 08:02, Baolin Wang wrote: > Anonymous pages have already been supported for multi-size (mTHP) allocation > through commit 19eaf44954df, that can allow THP to be configured through the > sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. > > However, the anonymous shared pages will ignore the anonymous mTHP rule > configured through the sysfs interface, and can only use the PMD-mapped > THP, that is not reasonable. Many implement anonymous page sharing through > mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, > therefore, users expect to apply an unified mTHP strategy for anonymous pages, > also including the anonymous shared pages, in order to enjoy the benefits of > mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat > than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. This sounds like a very useful addition! Out of interest, can you point me at any workloads (and off-the-shelf benchmarks for those workloads) that predominantly use shared anon memory? > > The primary strategy is that, the use of huge pages for anonymous shared pages > still follows the global control determined by the mount option "huge=" parameter > or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. > The utilization of mTHP is allowed only when the global 'huge' switch is enabled. > Subsequently, the mTHP sysfs interface (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) > is checked to determine the mTHP size that can be used for large folio allocation > for these anonymous shared pages. I'm not sure about this proposed control mechanism; won't it break compatibility? I could be wrong, but I don't think shmem's use of THP used to depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it doesn't make sense to me that we now depend upon the /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by default disables all sizes except 2M, which is set to "inherit" from /sys/kernel/mm/transparent_hugepage/enabled). The other problem is that shmem_enabled has a different set of options (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) Perhaps it would be cleaner to do the same trick we did for enabled; Introduce /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, plus the additional "inherit" option. By default all sizes will be set to "never" except 2M, which is set to "inherit". Of course the huge= mount option would also need to take a per-size option in this case. e.g. huge=2048kB:advise,64kB:always > > TODO: > - More testing and provide some performance data. > - Need more discussion about the large folio allocation strategy for a 'regular > file' operation created by memfd_create(), for example using ftruncate(fd) to specify > the 'file' size, which need to follow the anonymous mTHP rule too? > - Do not split the large folio when share memory swap out. > - Can swap in a large folio for share memory. > > Baolin Wang (5): > mm: memory: extend finish_fault() to support large folio > mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() > mm: shmem: add THP validation for PMD-mapped THP related statistics > mm: shmem: add mTHP support for anonymous share pages > mm: shmem: add anonymous share mTHP counters > > include/linux/huge_mm.h | 4 +- > mm/huge_memory.c | 8 ++- > mm/memory.c | 25 +++++++--- > mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- > 4 files changed, 108 insertions(+), 36 deletions(-) >
On 23.04.24 12:41, Ryan Roberts wrote: > On 22/04/2024 08:02, Baolin Wang wrote: >> Anonymous pages have already been supported for multi-size (mTHP) allocation >> through commit 19eaf44954df, that can allow THP to be configured through the >> sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >> >> However, the anonymous shared pages will ignore the anonymous mTHP rule >> configured through the sysfs interface, and can only use the PMD-mapped >> THP, that is not reasonable. Many implement anonymous page sharing through >> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >> also including the anonymous shared pages, in order to enjoy the benefits of >> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. > > This sounds like a very useful addition! > > Out of interest, can you point me at any workloads (and off-the-shelf benchmarks > for those workloads) that predominantly use shared anon memory? > >> >> The primary strategy is that, the use of huge pages for anonymous shared pages >> still follows the global control determined by the mount option "huge=" parameter >> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >> The utilization of mTHP is allowed only when the global 'huge' switch is enabled. >> Subsequently, the mTHP sysfs interface (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >> is checked to determine the mTHP size that can be used for large folio allocation >> for these anonymous shared pages. > > I'm not sure about this proposed control mechanism; won't it break > compatibility? I could be wrong, but I don't think shmem's use of THP used to > depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it > doesn't make sense to me that we now depend upon the > /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by > default disables all sizes except 2M, which is set to "inherit" from > /sys/kernel/mm/transparent_hugepage/enabled). > > The other problem is that shmem_enabled has a different set of options > (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) > > Perhaps it would be cleaner to do the same trick we did for enabled; Introduce > /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the > same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, > plus the additional "inherit" option. By default all sizes will be set to > "never" except 2M, which is set to "inherit". Matches what I had in mind.
On 2024/4/23 18:41, Ryan Roberts wrote: > On 22/04/2024 08:02, Baolin Wang wrote: >> Anonymous pages have already been supported for multi-size (mTHP) allocation >> through commit 19eaf44954df, that can allow THP to be configured through the >> sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >> >> However, the anonymous shared pages will ignore the anonymous mTHP rule >> configured through the sysfs interface, and can only use the PMD-mapped >> THP, that is not reasonable. Many implement anonymous page sharing through >> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >> also including the anonymous shared pages, in order to enjoy the benefits of >> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. > > This sounds like a very useful addition! > > Out of interest, can you point me at any workloads (and off-the-shelf benchmarks > for those workloads) that predominantly use shared anon memory? As far as I know, some database related workloads make extensive use of shared anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also uses shared anonymous memory. And I still need to do some investigation to measure the performance. [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >> The primary strategy is that, the use of huge pages for anonymous shared pages >> still follows the global control determined by the mount option "huge=" parameter >> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >> The utilization of mTHP is allowed only when the global 'huge' switch is enabled. >> Subsequently, the mTHP sysfs interface (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >> is checked to determine the mTHP size that can be used for large folio allocation >> for these anonymous shared pages. > > I'm not sure about this proposed control mechanism; won't it break > compatibility? I could be wrong, but I don't think shmem's use of THP used to > depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it Yes, I realized this after more testing. > doesn't make sense to me that we now depend upon the > /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by > default disables all sizes except 2M, which is set to "inherit" from > /sys/kernel/mm/transparent_hugepage/enabled). > > The other problem is that shmem_enabled has a different set of options > (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) > > Perhaps it would be cleaner to do the same trick we did for enabled; Introduce > /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the > same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, > plus the additional "inherit" option. By default all sizes will be set to > "never" except 2M, which is set to "inherit". Sounds good to me. But I do not want to copy all same values from top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled': always within_size advise never deny force For mTHP's shmem_enabled interface, we can just keep below values: always within_size advise never Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to 'never', and 'force' is equal to 'always'. > Of course the huge= mount option would also need to take a per-size option in > this case. e.g. huge=2048kB:advise,64kB:always IMO, I do not want to change the global 'huge=' mount option, which can control both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's keep it be same with the global sysfs interface: /sys/kernel/mm/transparent_hugepage/shmem_enabled. For tmpfs large folio strategy, I plan to address it later, and we may need more discussion to determine if it should follow the file large folio strategy or not (no investigation now). Thanks for reviewing. >> TODO: >> - More testing and provide some performance data. >> - Need more discussion about the large folio allocation strategy for a 'regular >> file' operation created by memfd_create(), for example using ftruncate(fd) to specify >> the 'file' size, which need to follow the anonymous mTHP rule too? >> - Do not split the large folio when share memory swap out. >> - Can swap in a large folio for share memory. >> >> Baolin Wang (5): >> mm: memory: extend finish_fault() to support large folio >> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >> mm: shmem: add THP validation for PMD-mapped THP related statistics >> mm: shmem: add mTHP support for anonymous share pages >> mm: shmem: add anonymous share mTHP counters >> >> include/linux/huge_mm.h | 4 +- >> mm/huge_memory.c | 8 ++- >> mm/memory.c | 25 +++++++--- >> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >> 4 files changed, 108 insertions(+), 36 deletions(-) >>
On 24/04/2024 07:55, Baolin Wang wrote: > > > On 2024/4/23 18:41, Ryan Roberts wrote: >> On 22/04/2024 08:02, Baolin Wang wrote: >>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>> through commit 19eaf44954df, that can allow THP to be configured through the >>> sysfs interface located at >>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>> >>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>> configured through the sysfs interface, and can only use the PMD-mapped >>> THP, that is not reasonable. Many implement anonymous page sharing through >>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >>> also including the anonymous shared pages, in order to enjoy the benefits of >>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. >> >> This sounds like a very useful addition! >> >> Out of interest, can you point me at any workloads (and off-the-shelf benchmarks >> for those workloads) that predominantly use shared anon memory? > > As far as I know, some database related workloads make extensive use of shared > anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also > uses shared anonymous memory. And I still need to do some investigation to > measure the performance. > > [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL Thanks for the pointer! > >>> The primary strategy is that, the use of huge pages for anonymous shared pages >>> still follows the global control determined by the mount option "huge=" >>> parameter >>> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>> The utilization of mTHP is allowed only when the global 'huge' switch is >>> enabled. >>> Subsequently, the mTHP sysfs interface >>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>> is checked to determine the mTHP size that can be used for large folio >>> allocation >>> for these anonymous shared pages. >> >> I'm not sure about this proposed control mechanism; won't it break >> compatibility? I could be wrong, but I don't think shmem's use of THP used to >> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it > > Yes, I realized this after more testing. > >> doesn't make sense to me that we now depend upon the >> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >> default disables all sizes except 2M, which is set to "inherit" from >> /sys/kernel/mm/transparent_hugepage/enabled). >> >> The other problem is that shmem_enabled has a different set of options >> (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) >> >> Perhaps it would be cleaner to do the same trick we did for enabled; Introduce >> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >> same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, >> plus the additional "inherit" option. By default all sizes will be set to >> "never" except 2M, which is set to "inherit". > > Sounds good to me. But I do not want to copy all same values from top-level > '/sys/kernel/mm/transparent_hugepage/shmem_enabled': > always within_size advise never deny force > > For mTHP's shmem_enabled interface, we can just keep below values: > always within_size advise never > > Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to > 'never', and 'force' is equal to 'always'. I'll admit it wasn't completely clear to me after reading the docs, but my rough understanding is: - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls mmap(SHARED|ANON) allocations (mostly; see rule 3) - huge=... controls tmpfs allocations - deny and force in shmem_enabled are equivalent to never and always for mmap(SHARED|ANON) but additionally override all tmpfs mounts so they act as if they were mounted with huge=never or huge=always Is that correct? If so, then I think it still makes sense to support per-size deny/force. Certainly if a per-size control is set to "inherit" and the top-level control is set to deny or force, you would need that to mean something. > >> Of course the huge= mount option would also need to take a per-size option in >> this case. e.g. huge=2048kB:advise,64kB:always > > IMO, I do not want to change the global 'huge=' mount option, which can control > both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's How does huge= control anon shmem? I thought it was only for mounted filesystems; so tmpfs? Perhaps my mental model for how this works is broken... > keep it be same with the global sysfs interface: > /sys/kernel/mm/transparent_hugepage/shmem_enabled. > > For tmpfs large folio strategy, I plan to address it later, and we may need more > discussion to determine if it should follow the file large folio strategy or not > (no investigation now). OK. But until you get to tmpfs, you'll need an interim definition for what it means if a per-size control is set to "inherit" and the top-level control is set to deny/force. > > Thanks for reviewing. No problem! Thanks for doing the work! > >>> TODO: >>> - More testing and provide some performance data. >>> - Need more discussion about the large folio allocation strategy for a >>> 'regular >>> file' operation created by memfd_create(), for example using ftruncate(fd) to >>> specify >>> the 'file' size, which need to follow the anonymous mTHP rule too? >>> - Do not split the large folio when share memory swap out. >>> - Can swap in a large folio for share memory. >>> >>> Baolin Wang (5): >>> mm: memory: extend finish_fault() to support large folio >>> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >>> mm: shmem: add THP validation for PMD-mapped THP related statistics >>> mm: shmem: add mTHP support for anonymous share pages >>> mm: shmem: add anonymous share mTHP counters >>> >>> include/linux/huge_mm.h | 4 +- >>> mm/huge_memory.c | 8 ++- >>> mm/memory.c | 25 +++++++--- >>> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >>> 4 files changed, 108 insertions(+), 36 deletions(-) >>>
On 2024/4/24 16:26, Ryan Roberts wrote: > On 24/04/2024 07:55, Baolin Wang wrote: >> >> >> On 2024/4/23 18:41, Ryan Roberts wrote: >>> On 22/04/2024 08:02, Baolin Wang wrote: >>>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>>> through commit 19eaf44954df, that can allow THP to be configured through the >>>> sysfs interface located at >>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>> >>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>> configured through the sysfs interface, and can only use the PMD-mapped >>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >>>> also including the anonymous shared pages, in order to enjoy the benefits of >>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. >>> >>> This sounds like a very useful addition! >>> >>> Out of interest, can you point me at any workloads (and off-the-shelf benchmarks >>> for those workloads) that predominantly use shared anon memory? >> >> As far as I know, some database related workloads make extensive use of shared >> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also >> uses shared anonymous memory. And I still need to do some investigation to >> measure the performance. >> >> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL > > Thanks for the pointer! > >> >>>> The primary strategy is that, the use of huge pages for anonymous shared pages >>>> still follows the global control determined by the mount option "huge=" >>>> parameter >>>> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>> enabled. >>>> Subsequently, the mTHP sysfs interface >>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>> is checked to determine the mTHP size that can be used for large folio >>>> allocation >>>> for these anonymous shared pages. >>> >>> I'm not sure about this proposed control mechanism; won't it break >>> compatibility? I could be wrong, but I don't think shmem's use of THP used to >>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >> >> Yes, I realized this after more testing. >> >>> doesn't make sense to me that we now depend upon the >>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>> default disables all sizes except 2M, which is set to "inherit" from >>> /sys/kernel/mm/transparent_hugepage/enabled). >>> >>> The other problem is that shmem_enabled has a different set of options >>> (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) >>> >>> Perhaps it would be cleaner to do the same trick we did for enabled; Introduce >>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >>> same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>> plus the additional "inherit" option. By default all sizes will be set to >>> "never" except 2M, which is set to "inherit". >> >> Sounds good to me. But I do not want to copy all same values from top-level >> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >> always within_size advise never deny force >> >> For mTHP's shmem_enabled interface, we can just keep below values: >> always within_size advise never >> >> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >> 'never', and 'force' is equal to 'always'. > > I'll admit it wasn't completely clear to me after reading the docs, but my rough > understanding is: > > - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls > mmap(SHARED|ANON) allocations (mostly; see rule 3) > - huge=... controls tmpfs allocations > - deny and force in shmem_enabled are equivalent to never and always for > mmap(SHARED|ANON) but additionally override all tmpfs mounts so they act as > if they were mounted with huge=never or huge=always > > Is that correct? If so, then I think it still makes sense to support per-size Correct. > deny/force. Certainly if a per-size control is set to "inherit" and the > top-level control is set to deny or force, you would need that to mean something. IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface should only control the anonymous shmem. And 'huge=' controls tmpfs allocation, so we should not use anonymous control to override tmpfs control, which seems a little mess? >>> Of course the huge= mount option would also need to take a per-size option in >>> this case. e.g. huge=2048kB:advise,64kB:always >> >> IMO, I do not want to change the global 'huge=' mount option, which can control >> both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's > > How does huge= control anon shmem? I thought it was only for mounted > filesystems; so tmpfs? Perhaps my mental model for how this works is broken... Sorry for noise, you are right. So this is still the reason I don't want to change the semantics of 'huge=', which is used to control tmpfs. >> keep it be same with the global sysfs interface: >> /sys/kernel/mm/transparent_hugepage/shmem_enabled. >> >> For tmpfs large folio strategy, I plan to address it later, and we may need more >> discussion to determine if it should follow the file large folio strategy or not >> (no investigation now). > > OK. But until you get to tmpfs, you'll need an interim definition for what it > means if a per-size control is set to "inherit" and the top-level control is set > to deny/force. > >> >> Thanks for reviewing. > > No problem! Thanks for doing the work! > >> >>>> TODO: >>>> - More testing and provide some performance data. >>>> - Need more discussion about the large folio allocation strategy for a >>>> 'regular >>>> file' operation created by memfd_create(), for example using ftruncate(fd) to >>>> specify >>>> the 'file' size, which need to follow the anonymous mTHP rule too? >>>> - Do not split the large folio when share memory swap out. >>>> - Can swap in a large folio for share memory. >>>> >>>> Baolin Wang (5): >>>> mm: memory: extend finish_fault() to support large folio >>>> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >>>> mm: shmem: add THP validation for PMD-mapped THP related statistics >>>> mm: shmem: add mTHP support for anonymous share pages >>>> mm: shmem: add anonymous share mTHP counters >>>> >>>> include/linux/huge_mm.h | 4 +- >>>> mm/huge_memory.c | 8 ++- >>>> mm/memory.c | 25 +++++++--- >>>> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >>>> 4 files changed, 108 insertions(+), 36 deletions(-) >>>>
On 24/04/2024 10:55, Baolin Wang wrote: > > > On 2024/4/24 16:26, Ryan Roberts wrote: >> On 24/04/2024 07:55, Baolin Wang wrote: >>> >>> >>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>>>> through commit 19eaf44954df, that can allow THP to be configured through the >>>>> sysfs interface located at >>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>> >>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >>>>> also including the anonymous shared pages, in order to enjoy the benefits of >>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss >>>>> etc. >>>> >>>> This sounds like a very useful addition! >>>> >>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>> benchmarks >>>> for those workloads) that predominantly use shared anon memory? >>> >>> As far as I know, some database related workloads make extensive use of shared >>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also >>> uses shared anonymous memory. And I still need to do some investigation to >>> measure the performance. >>> >>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >> >> Thanks for the pointer! >> >>> >>>>> The primary strategy is that, the use of huge pages for anonymous shared pages >>>>> still follows the global control determined by the mount option "huge=" >>>>> parameter >>>>> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>> enabled. >>>>> Subsequently, the mTHP sysfs interface >>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>> is checked to determine the mTHP size that can be used for large folio >>>>> allocation >>>>> for these anonymous shared pages. >>>> >>>> I'm not sure about this proposed control mechanism; won't it break >>>> compatibility? I could be wrong, but I don't think shmem's use of THP used to >>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>> >>> Yes, I realized this after more testing. >>> >>>> doesn't make sense to me that we now depend upon the >>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>> default disables all sizes except 2M, which is set to "inherit" from >>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>> >>>> The other problem is that shmem_enabled has a different set of options >>>> (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) >>>> >>>> Perhaps it would be cleaner to do the same trick we did for enabled; Introduce >>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >>>> same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>> plus the additional "inherit" option. By default all sizes will be set to >>>> "never" except 2M, which is set to "inherit". >>> >>> Sounds good to me. But I do not want to copy all same values from top-level >>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>> always within_size advise never deny force >>> >>> For mTHP's shmem_enabled interface, we can just keep below values: >>> always within_size advise never >>> >>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>> 'never', and 'force' is equal to 'always'. >> >> I'll admit it wasn't completely clear to me after reading the docs, but my rough >> understanding is: >> >> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >> mmap(SHARED|ANON) allocations (mostly; see rule 3) >> - huge=... controls tmpfs allocations >> - deny and force in shmem_enabled are equivalent to never and always for >> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they act as >> if they were mounted with huge=never or huge=always >> >> Is that correct? If so, then I think it still makes sense to support per-size > > Correct. > >> deny/force. Certainly if a per-size control is set to "inherit" and the >> top-level control is set to deny or force, you would need that to mean something. > > IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface > should only control the anonymous shmem. And 'huge=' controls tmpfs allocation, > so we should not use anonymous control to override tmpfs control, which seems a > little mess? I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave the tmpfs stuff for another time. But my point is that /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the value is deny or force. So if you have: echo deny > /mm/transparent_hugepage/shmem_enabled echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled What does that mean? > >>>> Of course the huge= mount option would also need to take a per-size option in >>>> this case. e.g. huge=2048kB:advise,64kB:always >>> >>> IMO, I do not want to change the global 'huge=' mount option, which can control >>> both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's >> >> How does huge= control anon shmem? I thought it was only for mounted >> filesystems; so tmpfs? Perhaps my mental model for how this works is broken... > > Sorry for noise, you are right. So this is still the reason I don't want to > change the semantics of 'huge=', which is used to control tmpfs. > >>> keep it be same with the global sysfs interface: >>> /sys/kernel/mm/transparent_hugepage/shmem_enabled. >>> >>> For tmpfs large folio strategy, I plan to address it later, and we may need more >>> discussion to determine if it should follow the file large folio strategy or not >>> (no investigation now). >> >> OK. But until you get to tmpfs, you'll need an interim definition for what it >> means if a per-size control is set to "inherit" and the top-level control is set >> to deny/force. >> >>> >>> Thanks for reviewing. >> >> No problem! Thanks for doing the work! >> >>> >>>>> TODO: >>>>> - More testing and provide some performance data. >>>>> - Need more discussion about the large folio allocation strategy for a >>>>> 'regular >>>>> file' operation created by memfd_create(), for example using ftruncate(fd) to >>>>> specify >>>>> the 'file' size, which need to follow the anonymous mTHP rule too? >>>>> - Do not split the large folio when share memory swap out. >>>>> - Can swap in a large folio for share memory. >>>>> >>>>> Baolin Wang (5): >>>>> mm: memory: extend finish_fault() to support large folio >>>>> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >>>>> mm: shmem: add THP validation for PMD-mapped THP related statistics >>>>> mm: shmem: add mTHP support for anonymous share pages >>>>> mm: shmem: add anonymous share mTHP counters >>>>> >>>>> include/linux/huge_mm.h | 4 +- >>>>> mm/huge_memory.c | 8 ++- >>>>> mm/memory.c | 25 +++++++--- >>>>> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >>>>> 4 files changed, 108 insertions(+), 36 deletions(-) >>>>>
On 2024/4/24 18:01, Ryan Roberts wrote: > On 24/04/2024 10:55, Baolin Wang wrote: >> >> >> On 2024/4/24 16:26, Ryan Roberts wrote: >>> On 24/04/2024 07:55, Baolin Wang wrote: >>>> >>>> >>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>>>>> through commit 19eaf44954df, that can allow THP to be configured through the >>>>>> sysfs interface located at >>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>> >>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous pages, >>>>>> also including the anonymous shared pages, in order to enjoy the benefits of >>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss >>>>>> etc. >>>>> >>>>> This sounds like a very useful addition! >>>>> >>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>> benchmarks >>>>> for those workloads) that predominantly use shared anon memory? >>>> >>>> As far as I know, some database related workloads make extensive use of shared >>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also >>>> uses shared anonymous memory. And I still need to do some investigation to >>>> measure the performance. >>>> >>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>> >>> Thanks for the pointer! >>> >>>> >>>>>> The primary strategy is that, the use of huge pages for anonymous shared pages >>>>>> still follows the global control determined by the mount option "huge=" >>>>>> parameter >>>>>> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>> enabled. >>>>>> Subsequently, the mTHP sysfs interface >>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>> allocation >>>>>> for these anonymous shared pages. >>>>> >>>>> I'm not sure about this proposed control mechanism; won't it break >>>>> compatibility? I could be wrong, but I don't think shmem's use of THP used to >>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>> >>>> Yes, I realized this after more testing. >>>> >>>>> doesn't make sense to me that we now depend upon the >>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>> >>>>> The other problem is that shmem_enabled has a different set of options >>>>> (always/never/within_size/advise/deny/force) to enabled (always/madvise/never) >>>>> >>>>> Perhaps it would be cleaner to do the same trick we did for enabled; Introduce >>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >>>>> same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>> "never" except 2M, which is set to "inherit". >>>> >>>> Sounds good to me. But I do not want to copy all same values from top-level >>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>> always within_size advise never deny force >>>> >>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>> always within_size advise never >>>> >>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>> 'never', and 'force' is equal to 'always'. >>> >>> I'll admit it wasn't completely clear to me after reading the docs, but my rough >>> understanding is: >>> >>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>> - huge=... controls tmpfs allocations >>> - deny and force in shmem_enabled are equivalent to never and always for >>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they act as >>> if they were mounted with huge=never or huge=always >>> >>> Is that correct? If so, then I think it still makes sense to support per-size >> >> Correct. >> >>> deny/force. Certainly if a per-size control is set to "inherit" and the >>> top-level control is set to deny or force, you would need that to mean something. >> >> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >> should only control the anonymous shmem. And 'huge=' controls tmpfs allocation, >> so we should not use anonymous control to override tmpfs control, which seems a >> little mess? > > I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave the > tmpfs stuff for another time. But my point is that > /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the > value is deny or force. So if you have: > > echo deny > /mm/transparent_hugepage/shmem_enabled IIUC, this global control will cause shmem_is_huge() to always return false, so no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, anonymous shmem will not use mTHP. No? > echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled > > What does that mean? > >> >>>>> Of course the huge= mount option would also need to take a per-size option in >>>>> this case. e.g. huge=2048kB:advise,64kB:always >>>> >>>> IMO, I do not want to change the global 'huge=' mount option, which can control >>>> both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's >>> >>> How does huge= control anon shmem? I thought it was only for mounted >>> filesystems; so tmpfs? Perhaps my mental model for how this works is broken... >> >> Sorry for noise, you are right. So this is still the reason I don't want to >> change the semantics of 'huge=', which is used to control tmpfs. >> >>>> keep it be same with the global sysfs interface: >>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled. >>>> >>>> For tmpfs large folio strategy, I plan to address it later, and we may need more >>>> discussion to determine if it should follow the file large folio strategy or not >>>> (no investigation now). >>> >>> OK. But until you get to tmpfs, you'll need an interim definition for what it >>> means if a per-size control is set to "inherit" and the top-level control is set >>> to deny/force. >>> >>>> >>>> Thanks for reviewing. >>> >>> No problem! Thanks for doing the work! >>> >>>> >>>>>> TODO: >>>>>> - More testing and provide some performance data. >>>>>> - Need more discussion about the large folio allocation strategy for a >>>>>> 'regular >>>>>> file' operation created by memfd_create(), for example using ftruncate(fd) to >>>>>> specify >>>>>> the 'file' size, which need to follow the anonymous mTHP rule too? >>>>>> - Do not split the large folio when share memory swap out. >>>>>> - Can swap in a large folio for share memory. >>>>>> >>>>>> Baolin Wang (5): >>>>>> mm: memory: extend finish_fault() to support large folio >>>>>> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >>>>>> mm: shmem: add THP validation for PMD-mapped THP related statistics >>>>>> mm: shmem: add mTHP support for anonymous share pages >>>>>> mm: shmem: add anonymous share mTHP counters >>>>>> >>>>>> include/linux/huge_mm.h | 4 +- >>>>>> mm/huge_memory.c | 8 ++- >>>>>> mm/memory.c | 25 +++++++--- >>>>>> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >>>>>> 4 files changed, 108 insertions(+), 36 deletions(-) >>>>>>
On 24/04/2024 14:49, Baolin Wang wrote: > > > On 2024/4/24 18:01, Ryan Roberts wrote: >> On 24/04/2024 10:55, Baolin Wang wrote: >>> >>> >>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>>>>>> through commit 19eaf44954df, that can allow THP to be configured through the >>>>>>> sysfs interface located at >>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>> >>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>> pages, >>>>>>> also including the anonymous shared pages, in order to enjoy the benefits of >>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss >>>>>>> etc. >>>>>> >>>>>> This sounds like a very useful addition! >>>>>> >>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>> benchmarks >>>>>> for those workloads) that predominantly use shared anon memory? >>>>> >>>>> As far as I know, some database related workloads make extensive use of shared >>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also >>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>> measure the performance. >>>>> >>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>> >>>> Thanks for the pointer! >>>> >>>>> >>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>> pages >>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>> parameter >>>>>>> or the sysfs interface at >>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>> enabled. >>>>>>> Subsequently, the mTHP sysfs interface >>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>> allocation >>>>>>> for these anonymous shared pages. >>>>>> >>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP used to >>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>>> >>>>> Yes, I realized this after more testing. >>>>> >>>>>> doesn't make sense to me that we now depend upon the >>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>> >>>>>> The other problem is that shmem_enabled has a different set of options >>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>> (always/madvise/never) >>>>>> >>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>> Introduce >>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >>>>>> same values as the top-level >>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>> "never" except 2M, which is set to "inherit". >>>>> >>>>> Sounds good to me. But I do not want to copy all same values from top-level >>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>> always within_size advise never deny force >>>>> >>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>> always within_size advise never >>>>> >>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>> 'never', and 'force' is equal to 'always'. >>>> >>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>> rough >>>> understanding is: >>>> >>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>> - huge=... controls tmpfs allocations >>>> - deny and force in shmem_enabled are equivalent to never and always for >>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>> act as >>>> if they were mounted with huge=never or huge=always >>>> >>>> Is that correct? If so, then I think it still makes sense to support per-size >>> >>> Correct. >>> >>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>> top-level control is set to deny or force, you would need that to mean >>>> something. >>> >>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>> should only control the anonymous shmem. And 'huge=' controls tmpfs allocation, >>> so we should not use anonymous control to override tmpfs control, which seems a >>> little mess? >> >> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave the >> tmpfs stuff for another time. But my point is that >> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >> value is deny or force. So if you have: >> >> echo deny > /mm/transparent_hugepage/shmem_enabled > > IIUC, this global control will cause shmem_is_huge() to always return false, so > no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, > anonymous shmem will not use mTHP. No? No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow the established pattern. For anon-private, each size is controlled by its /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is "inherit", in which case the value in /mm/transparent_hugepage/enabled is used for that size. That approach enables us to 1) maintain back-compat and 2) control each size independently 1) is met because the default is that all sizes are initially set to "never", except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) which is initially set to inherit. So any mTHP unaware SW can still modify /mm/transparent_hugepage/enabled and it will still only apply to PMD size. 2) is met because mTHP aware SW can come along and e.g. enable the 64K size (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to modify the value in /mm/transparent_hugepage/enabled. > >> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >> >> What does that mean? So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to support the deny and force values. When applied to non-PMD sizes, "deny" can just be a noop for now, because there was no way to configure a tmpfs mount for non-PMD size THP in the first place. But I'm not sure what to do with "force"? >> >>> >>>>>> Of course the huge= mount option would also need to take a per-size option in >>>>>> this case. e.g. huge=2048kB:advise,64kB:always >>>>> >>>>> IMO, I do not want to change the global 'huge=' mount option, which can >>>>> control >>>>> both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So >>>>> let's >>>> >>>> How does huge= control anon shmem? I thought it was only for mounted >>>> filesystems; so tmpfs? Perhaps my mental model for how this works is broken... >>> >>> Sorry for noise, you are right. So this is still the reason I don't want to >>> change the semantics of 'huge=', which is used to control tmpfs. >>> >>>>> keep it be same with the global sysfs interface: >>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled. >>>>> >>>>> For tmpfs large folio strategy, I plan to address it later, and we may need >>>>> more >>>>> discussion to determine if it should follow the file large folio strategy >>>>> or not >>>>> (no investigation now). >>>> >>>> OK. But until you get to tmpfs, you'll need an interim definition for what it >>>> means if a per-size control is set to "inherit" and the top-level control is >>>> set >>>> to deny/force. >>>> >>>>> >>>>> Thanks for reviewing. >>>> >>>> No problem! Thanks for doing the work! >>>> >>>>> >>>>>>> TODO: >>>>>>> - More testing and provide some performance data. >>>>>>> - Need more discussion about the large folio allocation strategy for a >>>>>>> 'regular >>>>>>> file' operation created by memfd_create(), for example using >>>>>>> ftruncate(fd) to >>>>>>> specify >>>>>>> the 'file' size, which need to follow the anonymous mTHP rule too? >>>>>>> - Do not split the large folio when share memory swap out. >>>>>>> - Can swap in a large folio for share memory. >>>>>>> >>>>>>> Baolin Wang (5): >>>>>>> mm: memory: extend finish_fault() to support large folio >>>>>>> mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio() >>>>>>> mm: shmem: add THP validation for PMD-mapped THP related statistics >>>>>>> mm: shmem: add mTHP support for anonymous share pages >>>>>>> mm: shmem: add anonymous share mTHP counters >>>>>>> >>>>>>> include/linux/huge_mm.h | 4 +- >>>>>>> mm/huge_memory.c | 8 ++- >>>>>>> mm/memory.c | 25 +++++++--- >>>>>>> mm/shmem.c | 107 ++++++++++++++++++++++++++++++---------- >>>>>>> 4 files changed, 108 insertions(+), 36 deletions(-) >>>>>>>
On 2024/4/24 22:20, Ryan Roberts wrote: > On 24/04/2024 14:49, Baolin Wang wrote: >> >> >> On 2024/4/24 18:01, Ryan Roberts wrote: >>> On 24/04/2024 10:55, Baolin Wang wrote: >>>> >>>> >>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) allocation >>>>>>>> through commit 19eaf44954df, that can allow THP to be configured through the >>>>>>>> sysfs interface located at >>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>> >>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>> pages, >>>>>>>> also including the anonymous shared pages, in order to enjoy the benefits of >>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss >>>>>>>> etc. >>>>>>> >>>>>>> This sounds like a very useful addition! >>>>>>> >>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>> benchmarks >>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>> >>>>>> As far as I know, some database related workloads make extensive use of shared >>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also >>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>> measure the performance. >>>>>> >>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>> >>>>> Thanks for the pointer! >>>>> >>>>>> >>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>> pages >>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>> parameter >>>>>>>> or the sysfs interface at >>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>> enabled. >>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>> allocation >>>>>>>> for these anonymous shared pages. >>>>>>> >>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP used to >>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>>>> >>>>>> Yes, I realized this after more testing. >>>>>> >>>>>>> doesn't make sense to me that we now depend upon the >>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>> >>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>> (always/madvise/never) >>>>>>> >>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>> Introduce >>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the >>>>>>> same values as the top-level >>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>> "never" except 2M, which is set to "inherit". >>>>>> >>>>>> Sounds good to me. But I do not want to copy all same values from top-level >>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>> always within_size advise never deny force >>>>>> >>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>> always within_size advise never >>>>>> >>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>> 'never', and 'force' is equal to 'always'. >>>>> >>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>> rough >>>>> understanding is: >>>>> >>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>> - huge=... controls tmpfs allocations >>>>> - deny and force in shmem_enabled are equivalent to never and always for >>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>> act as >>>>> if they were mounted with huge=never or huge=always >>>>> >>>>> Is that correct? If so, then I think it still makes sense to support per-size >>>> >>>> Correct. >>>> >>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>> top-level control is set to deny or force, you would need that to mean >>>>> something. >>>> >>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>> should only control the anonymous shmem. And 'huge=' controls tmpfs allocation, >>>> so we should not use anonymous control to override tmpfs control, which seems a >>>> little mess? >>> >>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave the >>> tmpfs stuff for another time. But my point is that >>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>> value is deny or force. So if you have: >>> >>> echo deny > /mm/transparent_hugepage/shmem_enabled >> >> IIUC, this global control will cause shmem_is_huge() to always return false, so >> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >> anonymous shmem will not use mTHP. No? > > No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and > I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow > the established pattern. > > For anon-private, each size is controlled by its > /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is > "inherit", in which case the value in /mm/transparent_hugepage/enabled is used > for that size. > > That approach enables us to 1) maintain back-compat and 2) control each size > independently > > 1) is met because the default is that all sizes are initially set to "never", > except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) > which is initially set to inherit. So any mTHP unaware SW can still modify > /mm/transparent_hugepage/enabled and it will still only apply to PMD size. > > 2) is met because mTHP aware SW can come along and e.g. enable the 64K size > (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to > modify the value in /mm/transparent_hugepage/enabled. Thanks for explanation. Initially, I want to make ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but I think it should follow the same strategy as anon mTHP as you said. >>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>> >>> What does that mean? > > So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to > support the deny and force values. When applied to non-PMD sizes, "deny" can > just be a noop for now, because there was no way to configure a tmpfs mount for > non-PMD size THP in the first place. But I'm not sure what to do with "force"? OK. And I also prefer that "force" should be a noop too, since anon shmem control should not configure tmpfs huge page allocation.
On 25/04/2024 07:20, Baolin Wang wrote: > > > On 2024/4/24 22:20, Ryan Roberts wrote: >> On 24/04/2024 14:49, Baolin Wang wrote: >>> >>> >>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) >>>>>>>>> allocation >>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured >>>>>>>>> through the >>>>>>>>> sysfs interface located at >>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>> >>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>>> pages, >>>>>>>>> also including the anonymous shared pages, in order to enjoy the >>>>>>>>> benefits of >>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB >>>>>>>>> miss >>>>>>>>> etc. >>>>>>>> >>>>>>>> This sounds like a very useful addition! >>>>>>>> >>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>>> benchmarks >>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>> >>>>>>> As far as I know, some database related workloads make extensive use of >>>>>>> shared >>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely >>>>>>> also >>>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>>> measure the performance. >>>>>>> >>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>> >>>>>> Thanks for the pointer! >>>>>> >>>>>>> >>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>>> pages >>>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>>> parameter >>>>>>>>> or the sysfs interface at >>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>>> enabled. >>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>>> allocation >>>>>>>>> for these anonymous shared pages. >>>>>>>> >>>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP >>>>>>>> used to >>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>>>>> >>>>>>> Yes, I realized this after more testing. >>>>>>> >>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>> >>>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>> (always/madvise/never) >>>>>>>> >>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>>> Introduce >>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all >>>>>>>> the >>>>>>>> same values as the top-level >>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>> >>>>>>> Sounds good to me. But I do not want to copy all same values from top-level >>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>> always within_size advise never deny force >>>>>>> >>>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>>> always within_size advise never >>>>>>> >>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>>> 'never', and 'force' is equal to 'always'. >>>>>> >>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>>> rough >>>>>> understanding is: >>>>>> >>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>> - huge=... controls tmpfs allocations >>>>>> - deny and force in shmem_enabled are equivalent to never and always for >>>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>>> act as >>>>>> if they were mounted with huge=never or huge=always >>>>>> >>>>>> Is that correct? If so, then I think it still makes sense to support per-size >>>>> >>>>> Correct. >>>>> >>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>>> top-level control is set to deny or force, you would need that to mean >>>>>> something. >>>>> >>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs >>>>> allocation, >>>>> so we should not use anonymous control to override tmpfs control, which >>>>> seems a >>>>> little mess? >>>> >>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave >>>> the >>>> tmpfs stuff for another time. But my point is that >>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>>> value is deny or force. So if you have: >>>> >>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>> >>> IIUC, this global control will cause shmem_is_huge() to always return false, so >>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>> anonymous shmem will not use mTHP. No? >> >> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow >> the established pattern. >> >> For anon-private, each size is controlled by its >> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is >> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used >> for that size. >> >> That approach enables us to 1) maintain back-compat and 2) control each size >> independently >> >> 1) is met because the default is that all sizes are initially set to "never", >> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) >> which is initially set to inherit. So any mTHP unaware SW can still modify >> /mm/transparent_hugepage/enabled and it will still only apply to PMD size. >> >> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size >> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to >> modify the value in /mm/transparent_hugepage/enabled. > > Thanks for explanation. Initially, I want to make > ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but > I think it should follow the same strategy as anon mTHP as you said. > >>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>> >>>> What does that mean? >> >> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to >> support the deny and force values. When applied to non-PMD sizes, "deny" can >> just be a noop for now, because there was no way to configure a tmpfs mount for >> non-PMD size THP in the first place. But I'm not sure what to do with "force"? > > OK. And I also prefer that "force" should be a noop too, since anon shmem > control should not configure tmpfs huge page allocation. I guess technically they won't be noops, but (for the non-PMD-sizes) "force" will be an alias for "always" and "deny" will be an alias for "never"? I was just a bit concerned about later changing that behavior to also impact tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it, I don't see that as a problem.
On 25.04.24 10:17, Ryan Roberts wrote: > On 25/04/2024 07:20, Baolin Wang wrote: >> >> >> On 2024/4/24 22:20, Ryan Roberts wrote: >>> On 24/04/2024 14:49, Baolin Wang wrote: >>>> >>>> >>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) >>>>>>>>>> allocation >>>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured >>>>>>>>>> through the >>>>>>>>>> sysfs interface located at >>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>> >>>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through >>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, >>>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>>>> pages, >>>>>>>>>> also including the anonymous shared pages, in order to enjoy the >>>>>>>>>> benefits of >>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat >>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB >>>>>>>>>> miss >>>>>>>>>> etc. >>>>>>>>> >>>>>>>>> This sounds like a very useful addition! >>>>>>>>> >>>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>>>> benchmarks >>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>> >>>>>>>> As far as I know, some database related workloads make extensive use of >>>>>>>> shared >>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely >>>>>>>> also >>>>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>>>> measure the performance. >>>>>>>> >>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>> >>>>>>> Thanks for the pointer! >>>>>>> >>>>>>>> >>>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>>>> pages >>>>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>>>> parameter >>>>>>>>>> or the sysfs interface at >>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>>>> enabled. >>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>>>> allocation >>>>>>>>>> for these anonymous shared pages. >>>>>>>>> >>>>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP >>>>>>>>> used to >>>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it >>>>>>>> >>>>>>>> Yes, I realized this after more testing. >>>>>>>> >>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by >>>>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>> >>>>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>> (always/madvise/never) >>>>>>>>> >>>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>>>> Introduce >>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all >>>>>>>>> the >>>>>>>>> same values as the top-level >>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>> >>>>>>>> Sounds good to me. But I do not want to copy all same values from top-level >>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>> always within_size advise never deny force >>>>>>>> >>>>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>>>> always within_size advise never >>>>>>>> >>>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>> >>>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>>>> rough >>>>>>> understanding is: >>>>>>> >>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>> - huge=... controls tmpfs allocations >>>>>>> - deny and force in shmem_enabled are equivalent to never and always for >>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>>>> act as >>>>>>> if they were mounted with huge=never or huge=always >>>>>>> >>>>>>> Is that correct? If so, then I think it still makes sense to support per-size >>>>>> >>>>>> Correct. >>>>>> >>>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>>>> top-level control is set to deny or force, you would need that to mean >>>>>>> something. >>>>>> >>>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs >>>>>> allocation, >>>>>> so we should not use anonymous control to override tmpfs control, which >>>>>> seems a >>>>>> little mess? >>>>> >>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave >>>>> the >>>>> tmpfs stuff for another time. But my point is that >>>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>>>> value is deny or force. So if you have: >>>>> >>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>> >>>> IIUC, this global control will cause shmem_is_huge() to always return false, so >>>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>> anonymous shmem will not use mTHP. No? >>> >>> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow >>> the established pattern. >>> >>> For anon-private, each size is controlled by its >>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is >>> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used >>> for that size. >>> >>> That approach enables us to 1) maintain back-compat and 2) control each size >>> independently >>> >>> 1) is met because the default is that all sizes are initially set to "never", >>> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) >>> which is initially set to inherit. So any mTHP unaware SW can still modify >>> /mm/transparent_hugepage/enabled and it will still only apply to PMD size. >>> >>> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size >>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to >>> modify the value in /mm/transparent_hugepage/enabled. >> >> Thanks for explanation. Initially, I want to make >> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but >> I think it should follow the same strategy as anon mTHP as you said. >> >>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>> >>>>> What does that mean? >>> >>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to >>> support the deny and force values. When applied to non-PMD sizes, "deny" can >>> just be a noop for now, because there was no way to configure a tmpfs mount for >>> non-PMD size THP in the first place. But I'm not sure what to do with "force"? >> >> OK. And I also prefer that "force" should be a noop too, since anon shmem >> control should not configure tmpfs huge page allocation. > > I guess technically they won't be noops, but (for the non-PMD-sizes) "force" > will be an alias for "always" and "deny" will be an alias for "never"? > > I was just a bit concerned about later changing that behavior to also impact > tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it, > I don't see that as a problem. Is the question what should happen if we "inherit" "force" or if someone specifies "force" for a mTP size explicitly?
On 25/04/2024 09:26, David Hildenbrand wrote: > On 25.04.24 10:17, Ryan Roberts wrote: >> On 25/04/2024 07:20, Baolin Wang wrote: >>> >>> >>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) >>>>>>>>>>> allocation >>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured >>>>>>>>>>> through the >>>>>>>>>>> sysfs interface located at >>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>> >>>>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing >>>>>>>>>>> through >>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>> scenarios, >>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>>>>> pages, >>>>>>>>>>> also including the anonymous shared pages, in order to enjoy the >>>>>>>>>>> benefits of >>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory >>>>>>>>>>> bloat >>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB >>>>>>>>>>> miss >>>>>>>>>>> etc. >>>>>>>>>> >>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>> >>>>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>>>>> benchmarks >>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>> >>>>>>>>> As far as I know, some database related workloads make extensive use of >>>>>>>>> shared >>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely >>>>>>>>> also >>>>>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>>>>> measure the performance. >>>>>>>>> >>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>> >>>>>>>> Thanks for the pointer! >>>>>>>> >>>>>>>>> >>>>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>>>>> pages >>>>>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>>>>> parameter >>>>>>>>>>> or the sysfs interface at >>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>>>>> enabled. >>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>>>>> allocation >>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>> >>>>>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP >>>>>>>>>> used to >>>>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>> So it >>>>>>>>> >>>>>>>>> Yes, I realized this after more testing. >>>>>>>>> >>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values >>>>>>>>>> (which by >>>>>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>> >>>>>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>> (always/madvise/never) >>>>>>>>>> >>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>>>>> Introduce >>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all >>>>>>>>>> the >>>>>>>>>> same values as the top-level >>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>> >>>>>>>>> Sounds good to me. But I do not want to copy all same values from >>>>>>>>> top-level >>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>> always within_size advise never deny force >>>>>>>>> >>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>>>>> always within_size advise never >>>>>>>>> >>>>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>> >>>>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>>>>> rough >>>>>>>> understanding is: >>>>>>>> >>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>> - huge=... controls tmpfs allocations >>>>>>>> - deny and force in shmem_enabled are equivalent to never and >>>>>>>> always for >>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>>>>> act as >>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>> >>>>>>>> Is that correct? If so, then I think it still makes sense to support >>>>>>>> per-size >>>>>>> >>>>>>> Correct. >>>>>>> >>>>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>>>>> top-level control is set to deny or force, you would need that to mean >>>>>>>> something. >>>>>>> >>>>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs >>>>>>> allocation, >>>>>>> so we should not use anonymous control to override tmpfs control, which >>>>>>> seems a >>>>>>> little mess? >>>>>> >>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave >>>>>> the >>>>>> tmpfs stuff for another time. But my point is that >>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>>>>> value is deny or force. So if you have: >>>>>> >>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>> >>>>> IIUC, this global control will cause shmem_is_huge() to always return >>>>> false, so >>>>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>> anonymous shmem will not use mTHP. No? >>>> >>>> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow >>>> the established pattern. >>>> >>>> For anon-private, each size is controlled by its >>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is >>>> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used >>>> for that size. >>>> >>>> That approach enables us to 1) maintain back-compat and 2) control each size >>>> independently >>>> >>>> 1) is met because the default is that all sizes are initially set to "never", >>>> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>> which is initially set to inherit. So any mTHP unaware SW can still modify >>>> /mm/transparent_hugepage/enabled and it will still only apply to PMD size. >>>> >>>> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size >>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without >>>> having to >>>> modify the value in /mm/transparent_hugepage/enabled. >>> >>> Thanks for explanation. Initially, I want to make >>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but >>> I think it should follow the same strategy as anon mTHP as you said. >>> >>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>> >>>>>> What does that mean? >>>> >>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to >>>> support the deny and force values. When applied to non-PMD sizes, "deny" can >>>> just be a noop for now, because there was no way to configure a tmpfs mount for >>>> non-PMD size THP in the first place. But I'm not sure what to do with "force"? >>> >>> OK. And I also prefer that "force" should be a noop too, since anon shmem >>> control should not configure tmpfs huge page allocation. >> >> I guess technically they won't be noops, but (for the non-PMD-sizes) "force" >> will be an alias for "always" and "deny" will be an alias for "never"? >> >> I was just a bit concerned about later changing that behavior to also impact >> tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it, >> I don't see that as a problem. > > Is the question what should happen if we "inherit" "force" or if someone > specifies "force" for a mTP size explicitly? Well I think it amounts to the same thing; there isn't much point in forbidding "force" to be set directly because it can still be set indirectly through "inherit". We can't forbid indirectly setting it, because "inherit" could be set first, then the top-level shmem_enabled changed to "force" after - and we wouldn't want to fail that. So I think the question is just 'what should happen when "force" is configured for a non-PMD-sized mTHP'? I think the answer is 'for now, it behaves like "always", but in future it will also force any tmpfs mounts to consider that mTHP size even if that size wasn't specified by mount='
On 25.04.24 10:46, Ryan Roberts wrote: > On 25/04/2024 09:26, David Hildenbrand wrote: >> On 25.04.24 10:17, Ryan Roberts wrote: >>> On 25/04/2024 07:20, Baolin Wang wrote: >>>> >>>> >>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP) >>>>>>>>>>>> allocation >>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured >>>>>>>>>>>> through the >>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>> >>>>>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule >>>>>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped >>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing >>>>>>>>>>>> through >>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>>> scenarios, >>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous >>>>>>>>>>>> pages, >>>>>>>>>>>> also including the anonymous shared pages, in order to enjoy the >>>>>>>>>>>> benefits of >>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory >>>>>>>>>>>> bloat >>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB >>>>>>>>>>>> miss >>>>>>>>>>>> etc. >>>>>>>>>>> >>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>> >>>>>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf >>>>>>>>>>> benchmarks >>>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>>> >>>>>>>>>> As far as I know, some database related workloads make extensive use of >>>>>>>>>> shared >>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely >>>>>>>>>> also >>>>>>>>>> uses shared anonymous memory. And I still need to do some investigation to >>>>>>>>>> measure the performance. >>>>>>>>>> >>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>> >>>>>>>>> Thanks for the pointer! >>>>>>>>> >>>>>>>>>> >>>>>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared >>>>>>>>>>>> pages >>>>>>>>>>>> still follows the global control determined by the mount option "huge=" >>>>>>>>>>>> parameter >>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is >>>>>>>>>>>> enabled. >>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>> is checked to determine the mTHP size that can be used for large folio >>>>>>>>>>>> allocation >>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>> >>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it break >>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP >>>>>>>>>>> used to >>>>>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>> So it >>>>>>>>>> >>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>> >>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values >>>>>>>>>>> (which by >>>>>>>>>>> default disables all sizes except 2M, which is set to "inherit" from >>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>> >>>>>>>>>>> The other problem is that shmem_enabled has a different set of options >>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>> (always/madvise/never) >>>>>>>>>>> >>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled; >>>>>>>>>>> Introduce >>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all >>>>>>>>>>> the >>>>>>>>>>> same values as the top-level >>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>> plus the additional "inherit" option. By default all sizes will be set to >>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>> >>>>>>>>>> Sounds good to me. But I do not want to copy all same values from >>>>>>>>>> top-level >>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>> always within_size advise never deny force >>>>>>>>>> >>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below values: >>>>>>>>>> always within_size advise never >>>>>>>>>> >>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to >>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>> >>>>>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my >>>>>>>>> rough >>>>>>>>> understanding is: >>>>>>>>> >>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls >>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>> - deny and force in shmem_enabled are equivalent to never and >>>>>>>>> always for >>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs mounts so they >>>>>>>>> act as >>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>> >>>>>>>>> Is that correct? If so, then I think it still makes sense to support >>>>>>>>> per-size >>>>>>>> >>>>>>>> Correct. >>>>>>>> >>>>>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the >>>>>>>>> top-level control is set to deny or force, you would need that to mean >>>>>>>>> something. >>>>>>>> >>>>>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs >>>>>>>> allocation, >>>>>>>> so we should not use anonymous control to override tmpfs control, which >>>>>>>> seems a >>>>>>>> little mess? >>>>>>> >>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave >>>>>>> the >>>>>>> tmpfs stuff for another time. But my point is that >>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the >>>>>>> value is deny or force. So if you have: >>>>>>> >>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>> >>>>>> IIUC, this global control will cause shmem_is_huge() to always return >>>>>> false, so >>>>>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>> anonymous shmem will not use mTHP. No? >>>>> >>>>> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow >>>>> the established pattern. >>>>> >>>>> For anon-private, each size is controlled by its >>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is >>>>> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used >>>>> for that size. >>>>> >>>>> That approach enables us to 1) maintain back-compat and 2) control each size >>>>> independently >>>>> >>>>> 1) is met because the default is that all sizes are initially set to "never", >>>>> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>> which is initially set to inherit. So any mTHP unaware SW can still modify >>>>> /mm/transparent_hugepage/enabled and it will still only apply to PMD size. >>>>> >>>>> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size >>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without >>>>> having to >>>>> modify the value in /mm/transparent_hugepage/enabled. >>>> >>>> Thanks for explanation. Initially, I want to make >>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but >>>> I think it should follow the same strategy as anon mTHP as you said. >>>> >>>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>> >>>>>>> What does that mean? >>>>> >>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to >>>>> support the deny and force values. When applied to non-PMD sizes, "deny" can >>>>> just be a noop for now, because there was no way to configure a tmpfs mount for >>>>> non-PMD size THP in the first place. But I'm not sure what to do with "force"? >>>> >>>> OK. And I also prefer that "force" should be a noop too, since anon shmem >>>> control should not configure tmpfs huge page allocation. >>> >>> I guess technically they won't be noops, but (for the non-PMD-sizes) "force" >>> will be an alias for "always" and "deny" will be an alias for "never"? >>> >>> I was just a bit concerned about later changing that behavior to also impact >>> tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it, >>> I don't see that as a problem. >> >> Is the question what should happen if we "inherit" "force" or if someone >> specifies "force" for a mTP size explicitly? > > Well I think it amounts to the same thing; there isn't much point in forbidding > "force" to be set directly because it can still be set indirectly through > "inherit". We can't forbid indirectly setting it, because "inherit" could be set > first, then the top-level shmem_enabled changed to "force" after - and we > wouldn't want to fail that. The default for PMD should be "inherit", for the other mTHP sizes it should be "never". So we should fail if: * Setting top-level to "force" when any non-PMD size is "inherit" * Setting "inherit" of a non-PMD size when the top-level is force Both will only happen if someone messes with the mTHP configuration manually. And we should only offer "force" as an option for PMD-sized mTHP as long as the others are not supported. See below. > > So I think the question is just 'what should happen when "force" is configured > for a non-PMD-sized mTHP'? We should hide it and not offer a configuration toggle that is inactive. If someone wants to sense support for other mTHP "force" settings in the future, they can just parse if the "shmem_enabled" toggle offers "force" as an option. Then they know that it can actually be enabled and will also do what is promised.
On 2024/4/25 16:57, David Hildenbrand wrote: > On 25.04.24 10:46, Ryan Roberts wrote: >> On 25/04/2024 09:26, David Hildenbrand wrote: >>> On 25.04.24 10:17, Ryan Roberts wrote: >>>> On 25/04/2024 07:20, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>>> Anonymous pages have already been supported for multi-size >>>>>>>>>>>>> (mTHP) >>>>>>>>>>>>> allocation >>>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be >>>>>>>>>>>>> configured >>>>>>>>>>>>> through the >>>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>>> >>>>>>>>>>>>> However, the anonymous shared pages will ignore the >>>>>>>>>>>>> anonymous mTHP rule >>>>>>>>>>>>> configured through the sysfs interface, and can only use >>>>>>>>>>>>> the PMD-mapped >>>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page >>>>>>>>>>>>> sharing >>>>>>>>>>>>> through >>>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>>>> scenarios, >>>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy >>>>>>>>>>>>> for anonymous >>>>>>>>>>>>> pages, >>>>>>>>>>>>> also including the anonymous shared pages, in order to >>>>>>>>>>>>> enjoy the >>>>>>>>>>>>> benefits of >>>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, >>>>>>>>>>>>> smaller memory >>>>>>>>>>>>> bloat >>>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to >>>>>>>>>>>>> reduce TLB >>>>>>>>>>>>> miss >>>>>>>>>>>>> etc. >>>>>>>>>>>> >>>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>>> >>>>>>>>>>>> Out of interest, can you point me at any workloads (and >>>>>>>>>>>> off-the-shelf >>>>>>>>>>>> benchmarks >>>>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>>>> >>>>>>>>>>> As far as I know, some database related workloads make >>>>>>>>>>> extensive use of >>>>>>>>>>> shared >>>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or >>>>>>>>>>> MySQL likely >>>>>>>>>>> also >>>>>>>>>>> uses shared anonymous memory. And I still need to do some >>>>>>>>>>> investigation to >>>>>>>>>>> measure the performance. >>>>>>>>>>> >>>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>>> >>>>>>>>>> Thanks for the pointer! >>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>>> The primary strategy is that, the use of huge pages for >>>>>>>>>>>>> anonymous shared >>>>>>>>>>>>> pages >>>>>>>>>>>>> still follows the global control determined by the mount >>>>>>>>>>>>> option "huge=" >>>>>>>>>>>>> parameter >>>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>>> The utilization of mTHP is allowed only when the global >>>>>>>>>>>>> 'huge' switch is >>>>>>>>>>>>> enabled. >>>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>>> is checked to determine the mTHP size that can be used for >>>>>>>>>>>>> large folio >>>>>>>>>>>>> allocation >>>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>>> >>>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it >>>>>>>>>>>> break >>>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's >>>>>>>>>>>> use of THP >>>>>>>>>>>> used to >>>>>>>>>>>> depend upon the value of >>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>>> So it >>>>>>>>>>> >>>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>>> >>>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled >>>>>>>>>>>> values >>>>>>>>>>>> (which by >>>>>>>>>>>> default disables all sizes except 2M, which is set to >>>>>>>>>>>> "inherit" from >>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>>> >>>>>>>>>>>> The other problem is that shmem_enabled has a different set >>>>>>>>>>>> of options >>>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>>> (always/madvise/never) >>>>>>>>>>>> >>>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for >>>>>>>>>>>> enabled; >>>>>>>>>>>> Introduce >>>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which >>>>>>>>>>>> can have all >>>>>>>>>>>> the >>>>>>>>>>>> same values as the top-level >>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>>> plus the additional "inherit" option. By default all sizes >>>>>>>>>>>> will be set to >>>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>>> >>>>>>>>>>> Sounds good to me. But I do not want to copy all same values >>>>>>>>>>> from >>>>>>>>>>> top-level >>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>>> always within_size advise never deny force >>>>>>>>>>> >>>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below >>>>>>>>>>> values: >>>>>>>>>>> always within_size advise never >>>>>>>>>>> >>>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, >>>>>>>>>>> 'deny' is equal to >>>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>>> >>>>>>>>>> I'll admit it wasn't completely clear to me after reading the >>>>>>>>>> docs, but my >>>>>>>>>> rough >>>>>>>>>> understanding is: >>>>>>>>>> >>>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled >>>>>>>>>> controls >>>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>>> - deny and force in shmem_enabled are equivalent to >>>>>>>>>> never and >>>>>>>>>> always for >>>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs >>>>>>>>>> mounts so they >>>>>>>>>> act as >>>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>>> >>>>>>>>>> Is that correct? If so, then I think it still makes sense to >>>>>>>>>> support >>>>>>>>>> per-size >>>>>>>>> >>>>>>>>> Correct. >>>>>>>>> >>>>>>>>>> deny/force. Certainly if a per-size control is set to >>>>>>>>>> "inherit" and the >>>>>>>>>> top-level control is set to deny or force, you would need that >>>>>>>>>> to mean >>>>>>>>>> something. >>>>>>>>> >>>>>>>>> IMHO, the >>>>>>>>> '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>>> should only control the anonymous shmem. And 'huge=' controls >>>>>>>>> tmpfs >>>>>>>>> allocation, >>>>>>>>> so we should not use anonymous control to override tmpfs >>>>>>>>> control, which >>>>>>>>> seems a >>>>>>>>> little mess? >>>>>>>> >>>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) >>>>>>>> here, and leave >>>>>>>> the >>>>>>>> tmpfs stuff for another time. But my point is that >>>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with >>>>>>>> tmpfs if the >>>>>>>> value is deny or force. So if you have: >>>>>>>> >>>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>>> >>>>>>> IIUC, this global control will cause shmem_is_huge() to always >>>>>>> return >>>>>>> false, so >>>>>>> no matter how >>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>>> anonymous shmem will not use mTHP. No? >>>>>> >>>>>> No, that's not how >>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' >>>>>> should follow >>>>>> the established pattern. >>>>>> >>>>>> For anon-private, each size is controlled by its >>>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that >>>>>> value is >>>>>> "inherit", in which case the value in >>>>>> /mm/transparent_hugepage/enabled is used >>>>>> for that size. >>>>>> >>>>>> That approach enables us to 1) maintain back-compat and 2) control >>>>>> each size >>>>>> independently >>>>>> >>>>>> 1) is met because the default is that all sizes are initially set >>>>>> to "never", >>>>>> except the PMD-size (e.g. >>>>>> /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>>> which is initially set to inherit. So any mTHP unaware SW can >>>>>> still modify >>>>>> /mm/transparent_hugepage/enabled and it will still only apply to >>>>>> PMD size. >>>>>> >>>>>> 2) is met because mTHP aware SW can come along and e.g. enable the >>>>>> 64K size >>>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) >>>>>> without >>>>>> having to >>>>>> modify the value in /mm/transparent_hugepage/enabled. >>>>> >>>>> Thanks for explanation. Initially, I want to make >>>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for >>>>> huge page, but >>>>> I think it should follow the same strategy as anon mTHP as you said. >>>>> >>>>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>>> >>>>>>>> What does that mean? >>>>>> >>>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled >>>>>> will need to >>>>>> support the deny and force values. When applied to non-PMD sizes, >>>>>> "deny" can >>>>>> just be a noop for now, because there was no way to configure a >>>>>> tmpfs mount for >>>>>> non-PMD size THP in the first place. But I'm not sure what to do >>>>>> with "force"? >>>>> >>>>> OK. And I also prefer that "force" should be a noop too, since anon >>>>> shmem >>>>> control should not configure tmpfs huge page allocation. >>>> >>>> I guess technically they won't be noops, but (for the non-PMD-sizes) >>>> "force" >>>> will be an alias for "always" and "deny" will be an alias for "never"? >>>> >>>> I was just a bit concerned about later changing that behavior to >>>> also impact >>>> tmpfs once tmpfs supports mTHP; could that cause breaks? But >>>> thinking about it, >>>> I don't see that as a problem. >>> >>> Is the question what should happen if we "inherit" "force" or if someone >>> specifies "force" for a mTP size explicitly? >> >> Well I think it amounts to the same thing; there isn't much point in >> forbidding >> "force" to be set directly because it can still be set indirectly through >> "inherit". We can't forbid indirectly setting it, because "inherit" >> could be set >> first, then the top-level shmem_enabled changed to "force" after - and we >> wouldn't want to fail that. > > The default for PMD should be "inherit", for the other mTHP sizes it > should be "never". > > So we should fail if: > * Setting top-level to "force" when any non-PMD size is "inherit" > * Setting "inherit" of a non-PMD size when the top-level is force IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not fail. So I think we should allow this configuration, but for tmpfs huge page allocation, we will not check the mTHP. > Both will only happen if someone messes with the mTHP configuration > manually. > > And we should only offer "force" as an option for PMD-sized mTHP as long > as the others are not supported. See below. > >> >> So I think the question is just 'what should happen when "force" is >> configured >> for a non-PMD-sized mTHP'? > > We should hide it and not offer a configuration toggle that is inactive. > > If someone wants to sense support for other mTHP "force" settings in the > future, they can just parse if the "shmem_enabled" toggle offers "force" > as an option. Then they know that it can actually be enabled and will > also do what is promised. Sounds good to me.
On 25.04.24 11:05, Baolin Wang wrote: > > > On 2024/4/25 16:57, David Hildenbrand wrote: >> On 25.04.24 10:46, Ryan Roberts wrote: >>> On 25/04/2024 09:26, David Hildenbrand wrote: >>>> On 25.04.24 10:17, Ryan Roberts wrote: >>>>> On 25/04/2024 07:20, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>>>> Anonymous pages have already been supported for multi-size >>>>>>>>>>>>>> (mTHP) >>>>>>>>>>>>>> allocation >>>>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be >>>>>>>>>>>>>> configured >>>>>>>>>>>>>> through the >>>>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>>>> >>>>>>>>>>>>>> However, the anonymous shared pages will ignore the >>>>>>>>>>>>>> anonymous mTHP rule >>>>>>>>>>>>>> configured through the sysfs interface, and can only use >>>>>>>>>>>>>> the PMD-mapped >>>>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page >>>>>>>>>>>>>> sharing >>>>>>>>>>>>>> through >>>>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>>>>> scenarios, >>>>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy >>>>>>>>>>>>>> for anonymous >>>>>>>>>>>>>> pages, >>>>>>>>>>>>>> also including the anonymous shared pages, in order to >>>>>>>>>>>>>> enjoy the >>>>>>>>>>>>>> benefits of >>>>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, >>>>>>>>>>>>>> smaller memory >>>>>>>>>>>>>> bloat >>>>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to >>>>>>>>>>>>>> reduce TLB >>>>>>>>>>>>>> miss >>>>>>>>>>>>>> etc. >>>>>>>>>>>>> >>>>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>>>> >>>>>>>>>>>>> Out of interest, can you point me at any workloads (and >>>>>>>>>>>>> off-the-shelf >>>>>>>>>>>>> benchmarks >>>>>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>>>>> >>>>>>>>>>>> As far as I know, some database related workloads make >>>>>>>>>>>> extensive use of >>>>>>>>>>>> shared >>>>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or >>>>>>>>>>>> MySQL likely >>>>>>>>>>>> also >>>>>>>>>>>> uses shared anonymous memory. And I still need to do some >>>>>>>>>>>> investigation to >>>>>>>>>>>> measure the performance. >>>>>>>>>>>> >>>>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>>>> >>>>>>>>>>> Thanks for the pointer! >>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>>> The primary strategy is that, the use of huge pages for >>>>>>>>>>>>>> anonymous shared >>>>>>>>>>>>>> pages >>>>>>>>>>>>>> still follows the global control determined by the mount >>>>>>>>>>>>>> option "huge=" >>>>>>>>>>>>>> parameter >>>>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>>>> The utilization of mTHP is allowed only when the global >>>>>>>>>>>>>> 'huge' switch is >>>>>>>>>>>>>> enabled. >>>>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>>>> is checked to determine the mTHP size that can be used for >>>>>>>>>>>>>> large folio >>>>>>>>>>>>>> allocation >>>>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>>>> >>>>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it >>>>>>>>>>>>> break >>>>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's >>>>>>>>>>>>> use of THP >>>>>>>>>>>>> used to >>>>>>>>>>>>> depend upon the value of >>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>>>> So it >>>>>>>>>>>> >>>>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>>>> >>>>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled >>>>>>>>>>>>> values >>>>>>>>>>>>> (which by >>>>>>>>>>>>> default disables all sizes except 2M, which is set to >>>>>>>>>>>>> "inherit" from >>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>>>> >>>>>>>>>>>>> The other problem is that shmem_enabled has a different set >>>>>>>>>>>>> of options >>>>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>>>> (always/madvise/never) >>>>>>>>>>>>> >>>>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for >>>>>>>>>>>>> enabled; >>>>>>>>>>>>> Introduce >>>>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which >>>>>>>>>>>>> can have all >>>>>>>>>>>>> the >>>>>>>>>>>>> same values as the top-level >>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>>>> plus the additional "inherit" option. By default all sizes >>>>>>>>>>>>> will be set to >>>>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>>>> >>>>>>>>>>>> Sounds good to me. But I do not want to copy all same values >>>>>>>>>>>> from >>>>>>>>>>>> top-level >>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>>>> always within_size advise never deny force >>>>>>>>>>>> >>>>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below >>>>>>>>>>>> values: >>>>>>>>>>>> always within_size advise never >>>>>>>>>>>> >>>>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, >>>>>>>>>>>> 'deny' is equal to >>>>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>>>> >>>>>>>>>>> I'll admit it wasn't completely clear to me after reading the >>>>>>>>>>> docs, but my >>>>>>>>>>> rough >>>>>>>>>>> understanding is: >>>>>>>>>>> >>>>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled >>>>>>>>>>> controls >>>>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>>>> - deny and force in shmem_enabled are equivalent to >>>>>>>>>>> never and >>>>>>>>>>> always for >>>>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs >>>>>>>>>>> mounts so they >>>>>>>>>>> act as >>>>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>>>> >>>>>>>>>>> Is that correct? If so, then I think it still makes sense to >>>>>>>>>>> support >>>>>>>>>>> per-size >>>>>>>>>> >>>>>>>>>> Correct. >>>>>>>>>> >>>>>>>>>>> deny/force. Certainly if a per-size control is set to >>>>>>>>>>> "inherit" and the >>>>>>>>>>> top-level control is set to deny or force, you would need that >>>>>>>>>>> to mean >>>>>>>>>>> something. >>>>>>>>>> >>>>>>>>>> IMHO, the >>>>>>>>>> '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>>>> should only control the anonymous shmem. And 'huge=' controls >>>>>>>>>> tmpfs >>>>>>>>>> allocation, >>>>>>>>>> so we should not use anonymous control to override tmpfs >>>>>>>>>> control, which >>>>>>>>>> seems a >>>>>>>>>> little mess? >>>>>>>>> >>>>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) >>>>>>>>> here, and leave >>>>>>>>> the >>>>>>>>> tmpfs stuff for another time. But my point is that >>>>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with >>>>>>>>> tmpfs if the >>>>>>>>> value is deny or force. So if you have: >>>>>>>>> >>>>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>>>> >>>>>>>> IIUC, this global control will cause shmem_is_huge() to always >>>>>>>> return >>>>>>>> false, so >>>>>>>> no matter how >>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>>>> anonymous shmem will not use mTHP. No? >>>>>>> >>>>>>> No, that's not how >>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' >>>>>>> should follow >>>>>>> the established pattern. >>>>>>> >>>>>>> For anon-private, each size is controlled by its >>>>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that >>>>>>> value is >>>>>>> "inherit", in which case the value in >>>>>>> /mm/transparent_hugepage/enabled is used >>>>>>> for that size. >>>>>>> >>>>>>> That approach enables us to 1) maintain back-compat and 2) control >>>>>>> each size >>>>>>> independently >>>>>>> >>>>>>> 1) is met because the default is that all sizes are initially set >>>>>>> to "never", >>>>>>> except the PMD-size (e.g. >>>>>>> /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>>>> which is initially set to inherit. So any mTHP unaware SW can >>>>>>> still modify >>>>>>> /mm/transparent_hugepage/enabled and it will still only apply to >>>>>>> PMD size. >>>>>>> >>>>>>> 2) is met because mTHP aware SW can come along and e.g. enable the >>>>>>> 64K size >>>>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) >>>>>>> without >>>>>>> having to >>>>>>> modify the value in /mm/transparent_hugepage/enabled. >>>>>> >>>>>> Thanks for explanation. Initially, I want to make >>>>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for >>>>>> huge page, but >>>>>> I think it should follow the same strategy as anon mTHP as you said. >>>>>> >>>>>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>>>> >>>>>>>>> What does that mean? >>>>>>> >>>>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled >>>>>>> will need to >>>>>>> support the deny and force values. When applied to non-PMD sizes, >>>>>>> "deny" can >>>>>>> just be a noop for now, because there was no way to configure a >>>>>>> tmpfs mount for >>>>>>> non-PMD size THP in the first place. But I'm not sure what to do >>>>>>> with "force"? >>>>>> >>>>>> OK. And I also prefer that "force" should be a noop too, since anon >>>>>> shmem >>>>>> control should not configure tmpfs huge page allocation. >>>>> >>>>> I guess technically they won't be noops, but (for the non-PMD-sizes) >>>>> "force" >>>>> will be an alias for "always" and "deny" will be an alias for "never"? >>>>> >>>>> I was just a bit concerned about later changing that behavior to >>>>> also impact >>>>> tmpfs once tmpfs supports mTHP; could that cause breaks? But >>>>> thinking about it, >>>>> I don't see that as a problem. >>>> >>>> Is the question what should happen if we "inherit" "force" or if someone >>>> specifies "force" for a mTP size explicitly? >>> >>> Well I think it amounts to the same thing; there isn't much point in >>> forbidding >>> "force" to be set directly because it can still be set indirectly through >>> "inherit". We can't forbid indirectly setting it, because "inherit" >>> could be set >>> first, then the top-level shmem_enabled changed to "force" after - and we >>> wouldn't want to fail that. >> >> The default for PMD should be "inherit", for the other mTHP sizes it >> should be "never". >> >> So we should fail if: >> * Setting top-level to "force" when any non-PMD size is "inherit" >> * Setting "inherit" of a non-PMD size when the top-level is force > > IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not > fail. If force does not apply to an mTHP size, it should fail if it would get inherited. Until it applies and we enable it. I'm still confused about all the toggles here, so could be I am missing something.
On 2024/4/25 17:20, David Hildenbrand wrote: > On 25.04.24 11:05, Baolin Wang wrote: >> >> >> On 2024/4/25 16:57, David Hildenbrand wrote: >>> On 25.04.24 10:46, Ryan Roberts wrote: >>>> On 25/04/2024 09:26, David Hildenbrand wrote: >>>>> On 25.04.24 10:17, Ryan Roberts wrote: >>>>>> On 25/04/2024 07:20, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>>>>> Anonymous pages have already been supported for multi-size >>>>>>>>>>>>>>> (mTHP) >>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be >>>>>>>>>>>>>>> configured >>>>>>>>>>>>>>> through the >>>>>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> However, the anonymous shared pages will ignore the >>>>>>>>>>>>>>> anonymous mTHP rule >>>>>>>>>>>>>>> configured through the sysfs interface, and can only use >>>>>>>>>>>>>>> the PMD-mapped >>>>>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page >>>>>>>>>>>>>>> sharing >>>>>>>>>>>>>>> through >>>>>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database >>>>>>>>>>>>>>> usage >>>>>>>>>>>>>>> scenarios, >>>>>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy >>>>>>>>>>>>>>> for anonymous >>>>>>>>>>>>>>> pages, >>>>>>>>>>>>>>> also including the anonymous shared pages, in order to >>>>>>>>>>>>>>> enjoy the >>>>>>>>>>>>>>> benefits of >>>>>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, >>>>>>>>>>>>>>> smaller memory >>>>>>>>>>>>>>> bloat >>>>>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to >>>>>>>>>>>>>>> reduce TLB >>>>>>>>>>>>>>> miss >>>>>>>>>>>>>>> etc. >>>>>>>>>>>>>> >>>>>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>>>>> >>>>>>>>>>>>>> Out of interest, can you point me at any workloads (and >>>>>>>>>>>>>> off-the-shelf >>>>>>>>>>>>>> benchmarks >>>>>>>>>>>>>> for those workloads) that predominantly use shared anon >>>>>>>>>>>>>> memory? >>>>>>>>>>>>> >>>>>>>>>>>>> As far as I know, some database related workloads make >>>>>>>>>>>>> extensive use of >>>>>>>>>>>>> shared >>>>>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or >>>>>>>>>>>>> MySQL likely >>>>>>>>>>>>> also >>>>>>>>>>>>> uses shared anonymous memory. And I still need to do some >>>>>>>>>>>>> investigation to >>>>>>>>>>>>> measure the performance. >>>>>>>>>>>>> >>>>>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>>>>> >>>>>>>>>>>> Thanks for the pointer! >>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>>>> The primary strategy is that, the use of huge pages for >>>>>>>>>>>>>>> anonymous shared >>>>>>>>>>>>>>> pages >>>>>>>>>>>>>>> still follows the global control determined by the mount >>>>>>>>>>>>>>> option "huge=" >>>>>>>>>>>>>>> parameter >>>>>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>>>>> The utilization of mTHP is allowed only when the global >>>>>>>>>>>>>>> 'huge' switch is >>>>>>>>>>>>>>> enabled. >>>>>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>>>>> is checked to determine the mTHP size that can be used for >>>>>>>>>>>>>>> large folio >>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>>>>> >>>>>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it >>>>>>>>>>>>>> break >>>>>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's >>>>>>>>>>>>>> use of THP >>>>>>>>>>>>>> used to >>>>>>>>>>>>>> depend upon the value of >>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>>>>> So it >>>>>>>>>>>>> >>>>>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>>>>> >>>>>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled >>>>>>>>>>>>>> values >>>>>>>>>>>>>> (which by >>>>>>>>>>>>>> default disables all sizes except 2M, which is set to >>>>>>>>>>>>>> "inherit" from >>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>>>>> >>>>>>>>>>>>>> The other problem is that shmem_enabled has a different set >>>>>>>>>>>>>> of options >>>>>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>>>>> (always/madvise/never) >>>>>>>>>>>>>> >>>>>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for >>>>>>>>>>>>>> enabled; >>>>>>>>>>>>>> Introduce >>>>>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which >>>>>>>>>>>>>> can have all >>>>>>>>>>>>>> the >>>>>>>>>>>>>> same values as the top-level >>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>>>>> plus the additional "inherit" option. By default all sizes >>>>>>>>>>>>>> will be set to >>>>>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>>>>> >>>>>>>>>>>>> Sounds good to me. But I do not want to copy all same values >>>>>>>>>>>>> from >>>>>>>>>>>>> top-level >>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>>>>> always within_size advise never deny force >>>>>>>>>>>>> >>>>>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below >>>>>>>>>>>>> values: >>>>>>>>>>>>> always within_size advise never >>>>>>>>>>>>> >>>>>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, >>>>>>>>>>>>> 'deny' is equal to >>>>>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>>>>> >>>>>>>>>>>> I'll admit it wasn't completely clear to me after reading the >>>>>>>>>>>> docs, but my >>>>>>>>>>>> rough >>>>>>>>>>>> understanding is: >>>>>>>>>>>> >>>>>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled >>>>>>>>>>>> controls >>>>>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>>>>> - deny and force in shmem_enabled are equivalent to >>>>>>>>>>>> never and >>>>>>>>>>>> always for >>>>>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs >>>>>>>>>>>> mounts so they >>>>>>>>>>>> act as >>>>>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>>>>> >>>>>>>>>>>> Is that correct? If so, then I think it still makes sense to >>>>>>>>>>>> support >>>>>>>>>>>> per-size >>>>>>>>>>> >>>>>>>>>>> Correct. >>>>>>>>>>> >>>>>>>>>>>> deny/force. Certainly if a per-size control is set to >>>>>>>>>>>> "inherit" and the >>>>>>>>>>>> top-level control is set to deny or force, you would need that >>>>>>>>>>>> to mean >>>>>>>>>>>> something. >>>>>>>>>>> >>>>>>>>>>> IMHO, the >>>>>>>>>>> '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>>>>> should only control the anonymous shmem. And 'huge=' controls >>>>>>>>>>> tmpfs >>>>>>>>>>> allocation, >>>>>>>>>>> so we should not use anonymous control to override tmpfs >>>>>>>>>>> control, which >>>>>>>>>>> seems a >>>>>>>>>>> little mess? >>>>>>>>>> >>>>>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) >>>>>>>>>> here, and leave >>>>>>>>>> the >>>>>>>>>> tmpfs stuff for another time. But my point is that >>>>>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with >>>>>>>>>> tmpfs if the >>>>>>>>>> value is deny or force. So if you have: >>>>>>>>>> >>>>>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>>>>> >>>>>>>>> IIUC, this global control will cause shmem_is_huge() to always >>>>>>>>> return >>>>>>>>> false, so >>>>>>>>> no matter how >>>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>>>>> anonymous shmem will not use mTHP. No? >>>>>>>> >>>>>>>> No, that's not how >>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' >>>>>>>> should follow >>>>>>>> the established pattern. >>>>>>>> >>>>>>>> For anon-private, each size is controlled by its >>>>>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that >>>>>>>> value is >>>>>>>> "inherit", in which case the value in >>>>>>>> /mm/transparent_hugepage/enabled is used >>>>>>>> for that size. >>>>>>>> >>>>>>>> That approach enables us to 1) maintain back-compat and 2) control >>>>>>>> each size >>>>>>>> independently >>>>>>>> >>>>>>>> 1) is met because the default is that all sizes are initially set >>>>>>>> to "never", >>>>>>>> except the PMD-size (e.g. >>>>>>>> /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>>>>> which is initially set to inherit. So any mTHP unaware SW can >>>>>>>> still modify >>>>>>>> /mm/transparent_hugepage/enabled and it will still only apply to >>>>>>>> PMD size. >>>>>>>> >>>>>>>> 2) is met because mTHP aware SW can come along and e.g. enable the >>>>>>>> 64K size >>>>>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) >>>>>>>> without >>>>>>>> having to >>>>>>>> modify the value in /mm/transparent_hugepage/enabled. >>>>>>> >>>>>>> Thanks for explanation. Initially, I want to make >>>>>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for >>>>>>> huge page, but >>>>>>> I think it should follow the same strategy as anon mTHP as you said. >>>>>>> >>>>>>>>>> echo inherit > >>>>>>>>>> /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>>>>> >>>>>>>>>> What does that mean? >>>>>>>> >>>>>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled >>>>>>>> will need to >>>>>>>> support the deny and force values. When applied to non-PMD sizes, >>>>>>>> "deny" can >>>>>>>> just be a noop for now, because there was no way to configure a >>>>>>>> tmpfs mount for >>>>>>>> non-PMD size THP in the first place. But I'm not sure what to do >>>>>>>> with "force"? >>>>>>> >>>>>>> OK. And I also prefer that "force" should be a noop too, since anon >>>>>>> shmem >>>>>>> control should not configure tmpfs huge page allocation. >>>>>> >>>>>> I guess technically they won't be noops, but (for the non-PMD-sizes) >>>>>> "force" >>>>>> will be an alias for "always" and "deny" will be an alias for >>>>>> "never"? >>>>>> >>>>>> I was just a bit concerned about later changing that behavior to >>>>>> also impact >>>>>> tmpfs once tmpfs supports mTHP; could that cause breaks? But >>>>>> thinking about it, >>>>>> I don't see that as a problem. >>>>> >>>>> Is the question what should happen if we "inherit" "force" or if >>>>> someone >>>>> specifies "force" for a mTP size explicitly? >>>> >>>> Well I think it amounts to the same thing; there isn't much point in >>>> forbidding >>>> "force" to be set directly because it can still be set indirectly >>>> through >>>> "inherit". We can't forbid indirectly setting it, because "inherit" >>>> could be set >>>> first, then the top-level shmem_enabled changed to "force" after - >>>> and we >>>> wouldn't want to fail that. >>> >>> The default for PMD should be "inherit", for the other mTHP sizes it >>> should be "never". >>> >>> So we should fail if: >>> * Setting top-level to "force" when any non-PMD size is "inherit" >>> * Setting "inherit" of a non-PMD size when the top-level is force >> >> IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not >> fail. > > If force does not apply to an mTHP size, it should fail if it would get > inherited. Until it applies and we enable it. > > I'm still confused about all the toggles here, so could be I am missing > something. Yes, this is a little messy:( After thinking more, considering that 'force' is used to override the tmpfs mount option, and 'inherit' will inherit the global setting. Your suggestion will make the logic eary to understand (though it is valid for anon shmem mTHP allocations, which are not part of this scenario), Ryan, what do you think?
On 25/04/2024 10:50, Baolin Wang wrote: > > > On 2024/4/25 17:20, David Hildenbrand wrote: >> On 25.04.24 11:05, Baolin Wang wrote: >>> >>> >>> On 2024/4/25 16:57, David Hildenbrand wrote: >>>> On 25.04.24 10:46, Ryan Roberts wrote: >>>>> On 25/04/2024 09:26, David Hildenbrand wrote: >>>>>> On 25.04.24 10:17, Ryan Roberts wrote: >>>>>>> On 25/04/2024 07:20, Baolin Wang wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>>>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>>>>>> Anonymous pages have already been supported for multi-size >>>>>>>>>>>>>>>> (mTHP) >>>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be >>>>>>>>>>>>>>>> configured >>>>>>>>>>>>>>>> through the >>>>>>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> However, the anonymous shared pages will ignore the >>>>>>>>>>>>>>>> anonymous mTHP rule >>>>>>>>>>>>>>>> configured through the sysfs interface, and can only use >>>>>>>>>>>>>>>> the PMD-mapped >>>>>>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page >>>>>>>>>>>>>>>> sharing >>>>>>>>>>>>>>>> through >>>>>>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>>>>>>> scenarios, >>>>>>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy >>>>>>>>>>>>>>>> for anonymous >>>>>>>>>>>>>>>> pages, >>>>>>>>>>>>>>>> also including the anonymous shared pages, in order to >>>>>>>>>>>>>>>> enjoy the >>>>>>>>>>>>>>>> benefits of >>>>>>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, >>>>>>>>>>>>>>>> smaller memory >>>>>>>>>>>>>>>> bloat >>>>>>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to >>>>>>>>>>>>>>>> reduce TLB >>>>>>>>>>>>>>>> miss >>>>>>>>>>>>>>>> etc. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Out of interest, can you point me at any workloads (and >>>>>>>>>>>>>>> off-the-shelf >>>>>>>>>>>>>>> benchmarks >>>>>>>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>>>>>>> >>>>>>>>>>>>>> As far as I know, some database related workloads make >>>>>>>>>>>>>> extensive use of >>>>>>>>>>>>>> shared >>>>>>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or >>>>>>>>>>>>>> MySQL likely >>>>>>>>>>>>>> also >>>>>>>>>>>>>> uses shared anonymous memory. And I still need to do some >>>>>>>>>>>>>> investigation to >>>>>>>>>>>>>> measure the performance. >>>>>>>>>>>>>> >>>>>>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>>>>>> >>>>>>>>>>>>> Thanks for the pointer! >>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>>>> The primary strategy is that, the use of huge pages for >>>>>>>>>>>>>>>> anonymous shared >>>>>>>>>>>>>>>> pages >>>>>>>>>>>>>>>> still follows the global control determined by the mount >>>>>>>>>>>>>>>> option "huge=" >>>>>>>>>>>>>>>> parameter >>>>>>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>>>>>> The utilization of mTHP is allowed only when the global >>>>>>>>>>>>>>>> 'huge' switch is >>>>>>>>>>>>>>>> enabled. >>>>>>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>>>>>> is checked to determine the mTHP size that can be used for >>>>>>>>>>>>>>>> large folio >>>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it >>>>>>>>>>>>>>> break >>>>>>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's >>>>>>>>>>>>>>> use of THP >>>>>>>>>>>>>>> used to >>>>>>>>>>>>>>> depend upon the value of >>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>>>>>> So it >>>>>>>>>>>>>> >>>>>>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>>>>>> >>>>>>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled >>>>>>>>>>>>>>> values >>>>>>>>>>>>>>> (which by >>>>>>>>>>>>>>> default disables all sizes except 2M, which is set to >>>>>>>>>>>>>>> "inherit" from >>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> The other problem is that shmem_enabled has a different set >>>>>>>>>>>>>>> of options >>>>>>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>>>>>> (always/madvise/never) >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for >>>>>>>>>>>>>>> enabled; >>>>>>>>>>>>>>> Introduce >>>>>>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which >>>>>>>>>>>>>>> can have all >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> same values as the top-level >>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>>>>>> plus the additional "inherit" option. By default all sizes >>>>>>>>>>>>>>> will be set to >>>>>>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>>>>>> >>>>>>>>>>>>>> Sounds good to me. But I do not want to copy all same values >>>>>>>>>>>>>> from >>>>>>>>>>>>>> top-level >>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>>>>>> always within_size advise never deny force >>>>>>>>>>>>>> >>>>>>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below >>>>>>>>>>>>>> values: >>>>>>>>>>>>>> always within_size advise never >>>>>>>>>>>>>> >>>>>>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, >>>>>>>>>>>>>> 'deny' is equal to >>>>>>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>>>>>> >>>>>>>>>>>>> I'll admit it wasn't completely clear to me after reading the >>>>>>>>>>>>> docs, but my >>>>>>>>>>>>> rough >>>>>>>>>>>>> understanding is: >>>>>>>>>>>>> >>>>>>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled >>>>>>>>>>>>> controls >>>>>>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>>>>>> - deny and force in shmem_enabled are equivalent to >>>>>>>>>>>>> never and >>>>>>>>>>>>> always for >>>>>>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs >>>>>>>>>>>>> mounts so they >>>>>>>>>>>>> act as >>>>>>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>>>>>> >>>>>>>>>>>>> Is that correct? If so, then I think it still makes sense to >>>>>>>>>>>>> support >>>>>>>>>>>>> per-size >>>>>>>>>>>> >>>>>>>>>>>> Correct. >>>>>>>>>>>> >>>>>>>>>>>>> deny/force. Certainly if a per-size control is set to >>>>>>>>>>>>> "inherit" and the >>>>>>>>>>>>> top-level control is set to deny or force, you would need that >>>>>>>>>>>>> to mean >>>>>>>>>>>>> something. >>>>>>>>>>>> >>>>>>>>>>>> IMHO, the >>>>>>>>>>>> '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>>>>>> should only control the anonymous shmem. And 'huge=' controls >>>>>>>>>>>> tmpfs >>>>>>>>>>>> allocation, >>>>>>>>>>>> so we should not use anonymous control to override tmpfs >>>>>>>>>>>> control, which >>>>>>>>>>>> seems a >>>>>>>>>>>> little mess? >>>>>>>>>>> >>>>>>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) >>>>>>>>>>> here, and leave >>>>>>>>>>> the >>>>>>>>>>> tmpfs stuff for another time. But my point is that >>>>>>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with >>>>>>>>>>> tmpfs if the >>>>>>>>>>> value is deny or force. So if you have: >>>>>>>>>>> >>>>>>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>>>>>> >>>>>>>>>> IIUC, this global control will cause shmem_is_huge() to always >>>>>>>>>> return >>>>>>>>>> false, so >>>>>>>>>> no matter how >>>>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>>>>>> anonymous shmem will not use mTHP. No? >>>>>>>>> >>>>>>>>> No, that's not how >>>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>>>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' >>>>>>>>> should follow >>>>>>>>> the established pattern. >>>>>>>>> >>>>>>>>> For anon-private, each size is controlled by its >>>>>>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that >>>>>>>>> value is >>>>>>>>> "inherit", in which case the value in >>>>>>>>> /mm/transparent_hugepage/enabled is used >>>>>>>>> for that size. >>>>>>>>> >>>>>>>>> That approach enables us to 1) maintain back-compat and 2) control >>>>>>>>> each size >>>>>>>>> independently >>>>>>>>> >>>>>>>>> 1) is met because the default is that all sizes are initially set >>>>>>>>> to "never", >>>>>>>>> except the PMD-size (e.g. >>>>>>>>> /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>>>>>> which is initially set to inherit. So any mTHP unaware SW can >>>>>>>>> still modify >>>>>>>>> /mm/transparent_hugepage/enabled and it will still only apply to >>>>>>>>> PMD size. >>>>>>>>> >>>>>>>>> 2) is met because mTHP aware SW can come along and e.g. enable the >>>>>>>>> 64K size >>>>>>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) >>>>>>>>> without >>>>>>>>> having to >>>>>>>>> modify the value in /mm/transparent_hugepage/enabled. >>>>>>>> >>>>>>>> Thanks for explanation. Initially, I want to make >>>>>>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for >>>>>>>> huge page, but >>>>>>>> I think it should follow the same strategy as anon mTHP as you said. >>>>>>>> >>>>>>>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>>>>>> >>>>>>>>>>> What does that mean? >>>>>>>>> >>>>>>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled >>>>>>>>> will need to >>>>>>>>> support the deny and force values. When applied to non-PMD sizes, >>>>>>>>> "deny" can >>>>>>>>> just be a noop for now, because there was no way to configure a >>>>>>>>> tmpfs mount for >>>>>>>>> non-PMD size THP in the first place. But I'm not sure what to do >>>>>>>>> with "force"? >>>>>>>> >>>>>>>> OK. And I also prefer that "force" should be a noop too, since anon >>>>>>>> shmem >>>>>>>> control should not configure tmpfs huge page allocation. >>>>>>> >>>>>>> I guess technically they won't be noops, but (for the non-PMD-sizes) >>>>>>> "force" >>>>>>> will be an alias for "always" and "deny" will be an alias for "never"? >>>>>>> >>>>>>> I was just a bit concerned about later changing that behavior to >>>>>>> also impact >>>>>>> tmpfs once tmpfs supports mTHP; could that cause breaks? But >>>>>>> thinking about it, >>>>>>> I don't see that as a problem. >>>>>> >>>>>> Is the question what should happen if we "inherit" "force" or if someone >>>>>> specifies "force" for a mTP size explicitly? >>>>> >>>>> Well I think it amounts to the same thing; there isn't much point in >>>>> forbidding >>>>> "force" to be set directly because it can still be set indirectly through >>>>> "inherit". We can't forbid indirectly setting it, because "inherit" >>>>> could be set >>>>> first, then the top-level shmem_enabled changed to "force" after - and we >>>>> wouldn't want to fail that. >>>> >>>> The default for PMD should be "inherit", for the other mTHP sizes it >>>> should be "never". >>>> >>>> So we should fail if: >>>> * Setting top-level to "force" when any non-PMD size is "inherit" >>>> * Setting "inherit" of a non-PMD size when the top-level is force >>> >>> IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not >>> fail. >> >> If force does not apply to an mTHP size, it should fail if it would get >> inherited. Until it applies and we enable it. >> >> I'm still confused about all the toggles here, so could be I am missing >> something. > > Yes, this is a little messy:( > > After thinking more, considering that 'force' is used to override the tmpfs > mount option, and 'inherit' will inherit the global setting. Your suggestion > will make the logic eary to understand (though it is valid for anon shmem mTHP > allocations, which are not part of this scenario), Ryan, what do you think? I was thinking that ever returning an error code when trying to set "force" for /mm/transparent_hugepage/shmem_enabled would be weird behavior and to be avoided if possible. But as always, David makes a clear and compelling case. I'm happy to go with his suggestion. I wonder if we should do the same for "deny"? The argument I previously made was that non-PMD-size mTHP is always effectively denied while tmpfs doesn't support mTHP, so we can allow "deny" and for now it would just be an alias for "never". The semantics don't change when you introduce tmpfs support. Personally I lean towards allowing "deny" from day 1. David?
On 25.04.24 12:17, Ryan Roberts wrote: > On 25/04/2024 10:50, Baolin Wang wrote: >> >> >> On 2024/4/25 17:20, David Hildenbrand wrote: >>> On 25.04.24 11:05, Baolin Wang wrote: >>>> >>>> >>>> On 2024/4/25 16:57, David Hildenbrand wrote: >>>>> On 25.04.24 10:46, Ryan Roberts wrote: >>>>>> On 25/04/2024 09:26, David Hildenbrand wrote: >>>>>>> On 25.04.24 10:17, Ryan Roberts wrote: >>>>>>>> On 25/04/2024 07:20, Baolin Wang wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> On 2024/4/24 22:20, Ryan Roberts wrote: >>>>>>>>>> On 24/04/2024 14:49, Baolin Wang wrote: >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On 2024/4/24 18:01, Ryan Roberts wrote: >>>>>>>>>>>> On 24/04/2024 10:55, Baolin Wang wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote: >>>>>>>>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote: >>>>>>>>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote: >>>>>>>>>>>>>>>>> Anonymous pages have already been supported for multi-size >>>>>>>>>>>>>>>>> (mTHP) >>>>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be >>>>>>>>>>>>>>>>> configured >>>>>>>>>>>>>>>>> through the >>>>>>>>>>>>>>>>> sysfs interface located at >>>>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> However, the anonymous shared pages will ignore the >>>>>>>>>>>>>>>>> anonymous mTHP rule >>>>>>>>>>>>>>>>> configured through the sysfs interface, and can only use >>>>>>>>>>>>>>>>> the PMD-mapped >>>>>>>>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page >>>>>>>>>>>>>>>>> sharing >>>>>>>>>>>>>>>>> through >>>>>>>>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage >>>>>>>>>>>>>>>>> scenarios, >>>>>>>>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy >>>>>>>>>>>>>>>>> for anonymous >>>>>>>>>>>>>>>>> pages, >>>>>>>>>>>>>>>>> also including the anonymous shared pages, in order to >>>>>>>>>>>>>>>>> enjoy the >>>>>>>>>>>>>>>>> benefits of >>>>>>>>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, >>>>>>>>>>>>>>>>> smaller memory >>>>>>>>>>>>>>>>> bloat >>>>>>>>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to >>>>>>>>>>>>>>>>> reduce TLB >>>>>>>>>>>>>>>>> miss >>>>>>>>>>>>>>>>> etc. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> This sounds like a very useful addition! >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Out of interest, can you point me at any workloads (and >>>>>>>>>>>>>>>> off-the-shelf >>>>>>>>>>>>>>>> benchmarks >>>>>>>>>>>>>>>> for those workloads) that predominantly use shared anon memory? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> As far as I know, some database related workloads make >>>>>>>>>>>>>>> extensive use of >>>>>>>>>>>>>>> shared >>>>>>>>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or >>>>>>>>>>>>>>> MySQL likely >>>>>>>>>>>>>>> also >>>>>>>>>>>>>>> uses shared anonymous memory. And I still need to do some >>>>>>>>>>>>>>> investigation to >>>>>>>>>>>>>>> measure the performance. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL >>>>>>>>>>>>>> >>>>>>>>>>>>>> Thanks for the pointer! >>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> The primary strategy is that, the use of huge pages for >>>>>>>>>>>>>>>>> anonymous shared >>>>>>>>>>>>>>>>> pages >>>>>>>>>>>>>>>>> still follows the global control determined by the mount >>>>>>>>>>>>>>>>> option "huge=" >>>>>>>>>>>>>>>>> parameter >>>>>>>>>>>>>>>>> or the sysfs interface at >>>>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'. >>>>>>>>>>>>>>>>> The utilization of mTHP is allowed only when the global >>>>>>>>>>>>>>>>> 'huge' switch is >>>>>>>>>>>>>>>>> enabled. >>>>>>>>>>>>>>>>> Subsequently, the mTHP sysfs interface >>>>>>>>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled) >>>>>>>>>>>>>>>>> is checked to determine the mTHP size that can be used for >>>>>>>>>>>>>>>>> large folio >>>>>>>>>>>>>>>>> allocation >>>>>>>>>>>>>>>>> for these anonymous shared pages. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> I'm not sure about this proposed control mechanism; won't it >>>>>>>>>>>>>>>> break >>>>>>>>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's >>>>>>>>>>>>>>>> use of THP >>>>>>>>>>>>>>>> used to >>>>>>>>>>>>>>>> depend upon the value of >>>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled? >>>>>>>>>>>>>>>> So it >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Yes, I realized this after more testing. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> doesn't make sense to me that we now depend upon the >>>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled >>>>>>>>>>>>>>>> values >>>>>>>>>>>>>>>> (which by >>>>>>>>>>>>>>>> default disables all sizes except 2M, which is set to >>>>>>>>>>>>>>>> "inherit" from >>>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled). >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> The other problem is that shmem_enabled has a different set >>>>>>>>>>>>>>>> of options >>>>>>>>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled >>>>>>>>>>>>>>>> (always/madvise/never) >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for >>>>>>>>>>>>>>>> enabled; >>>>>>>>>>>>>>>> Introduce >>>>>>>>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which >>>>>>>>>>>>>>>> can have all >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> same values as the top-level >>>>>>>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled, >>>>>>>>>>>>>>>> plus the additional "inherit" option. By default all sizes >>>>>>>>>>>>>>>> will be set to >>>>>>>>>>>>>>>> "never" except 2M, which is set to "inherit". >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Sounds good to me. But I do not want to copy all same values >>>>>>>>>>>>>>> from >>>>>>>>>>>>>>> top-level >>>>>>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled': >>>>>>>>>>>>>>> always within_size advise never deny force >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below >>>>>>>>>>>>>>> values: >>>>>>>>>>>>>>> always within_size advise never >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Cause when checking if mTHP can be used for anon shmem, >>>>>>>>>>>>>>> 'deny' is equal to >>>>>>>>>>>>>>> 'never', and 'force' is equal to 'always'. >>>>>>>>>>>>>> >>>>>>>>>>>>>> I'll admit it wasn't completely clear to me after reading the >>>>>>>>>>>>>> docs, but my >>>>>>>>>>>>>> rough >>>>>>>>>>>>>> understanding is: >>>>>>>>>>>>>> >>>>>>>>>>>>>> - /sys/kernel/mm/transparent_hugepage/shmem_enabled >>>>>>>>>>>>>> controls >>>>>>>>>>>>>> mmap(SHARED|ANON) allocations (mostly; see rule 3) >>>>>>>>>>>>>> - huge=... controls tmpfs allocations >>>>>>>>>>>>>> - deny and force in shmem_enabled are equivalent to >>>>>>>>>>>>>> never and >>>>>>>>>>>>>> always for >>>>>>>>>>>>>> mmap(SHARED|ANON) but additionally override all tmpfs >>>>>>>>>>>>>> mounts so they >>>>>>>>>>>>>> act as >>>>>>>>>>>>>> if they were mounted with huge=never or huge=always >>>>>>>>>>>>>> >>>>>>>>>>>>>> Is that correct? If so, then I think it still makes sense to >>>>>>>>>>>>>> support >>>>>>>>>>>>>> per-size >>>>>>>>>>>>> >>>>>>>>>>>>> Correct. >>>>>>>>>>>>> >>>>>>>>>>>>>> deny/force. Certainly if a per-size control is set to >>>>>>>>>>>>>> "inherit" and the >>>>>>>>>>>>>> top-level control is set to deny or force, you would need that >>>>>>>>>>>>>> to mean >>>>>>>>>>>>>> something. >>>>>>>>>>>>> >>>>>>>>>>>>> IMHO, the >>>>>>>>>>>>> '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface >>>>>>>>>>>>> should only control the anonymous shmem. And 'huge=' controls >>>>>>>>>>>>> tmpfs >>>>>>>>>>>>> allocation, >>>>>>>>>>>>> so we should not use anonymous control to override tmpfs >>>>>>>>>>>>> control, which >>>>>>>>>>>>> seems a >>>>>>>>>>>>> little mess? >>>>>>>>>>>> >>>>>>>>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) >>>>>>>>>>>> here, and leave >>>>>>>>>>>> the >>>>>>>>>>>> tmpfs stuff for another time. But my point is that >>>>>>>>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with >>>>>>>>>>>> tmpfs if the >>>>>>>>>>>> value is deny or force. So if you have: >>>>>>>>>>>> >>>>>>>>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled >>>>>>>>>>> >>>>>>>>>>> IIUC, this global control will cause shmem_is_huge() to always >>>>>>>>>>> return >>>>>>>>>>> false, so >>>>>>>>>>> no matter how >>>>>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set, >>>>>>>>>>> anonymous shmem will not use mTHP. No? >>>>>>>>>> >>>>>>>>>> No, that's not how >>>>>>>>>> '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and >>>>>>>>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' >>>>>>>>>> should follow >>>>>>>>>> the established pattern. >>>>>>>>>> >>>>>>>>>> For anon-private, each size is controlled by its >>>>>>>>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that >>>>>>>>>> value is >>>>>>>>>> "inherit", in which case the value in >>>>>>>>>> /mm/transparent_hugepage/enabled is used >>>>>>>>>> for that size. >>>>>>>>>> >>>>>>>>>> That approach enables us to 1) maintain back-compat and 2) control >>>>>>>>>> each size >>>>>>>>>> independently >>>>>>>>>> >>>>>>>>>> 1) is met because the default is that all sizes are initially set >>>>>>>>>> to "never", >>>>>>>>>> except the PMD-size (e.g. >>>>>>>>>> /mm/transparent_hugepage/hugepage-2048kB/enabled) >>>>>>>>>> which is initially set to inherit. So any mTHP unaware SW can >>>>>>>>>> still modify >>>>>>>>>> /mm/transparent_hugepage/enabled and it will still only apply to >>>>>>>>>> PMD size. >>>>>>>>>> >>>>>>>>>> 2) is met because mTHP aware SW can come along and e.g. enable the >>>>>>>>>> 64K size >>>>>>>>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) >>>>>>>>>> without >>>>>>>>>> having to >>>>>>>>>> modify the value in /mm/transparent_hugepage/enabled. >>>>>>>>> >>>>>>>>> Thanks for explanation. Initially, I want to make >>>>>>>>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for >>>>>>>>> huge page, but >>>>>>>>> I think it should follow the same strategy as anon mTHP as you said. >>>>>>>>> >>>>>>>>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled >>>>>>>>>>>> >>>>>>>>>>>> What does that mean? >>>>>>>>>> >>>>>>>>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled >>>>>>>>>> will need to >>>>>>>>>> support the deny and force values. When applied to non-PMD sizes, >>>>>>>>>> "deny" can >>>>>>>>>> just be a noop for now, because there was no way to configure a >>>>>>>>>> tmpfs mount for >>>>>>>>>> non-PMD size THP in the first place. But I'm not sure what to do >>>>>>>>>> with "force"? >>>>>>>>> >>>>>>>>> OK. And I also prefer that "force" should be a noop too, since anon >>>>>>>>> shmem >>>>>>>>> control should not configure tmpfs huge page allocation. >>>>>>>> >>>>>>>> I guess technically they won't be noops, but (for the non-PMD-sizes) >>>>>>>> "force" >>>>>>>> will be an alias for "always" and "deny" will be an alias for "never"? >>>>>>>> >>>>>>>> I was just a bit concerned about later changing that behavior to >>>>>>>> also impact >>>>>>>> tmpfs once tmpfs supports mTHP; could that cause breaks? But >>>>>>>> thinking about it, >>>>>>>> I don't see that as a problem. >>>>>>> >>>>>>> Is the question what should happen if we "inherit" "force" or if someone >>>>>>> specifies "force" for a mTP size explicitly? >>>>>> >>>>>> Well I think it amounts to the same thing; there isn't much point in >>>>>> forbidding >>>>>> "force" to be set directly because it can still be set indirectly through >>>>>> "inherit". We can't forbid indirectly setting it, because "inherit" >>>>>> could be set >>>>>> first, then the top-level shmem_enabled changed to "force" after - and we >>>>>> wouldn't want to fail that. >>>>> >>>>> The default for PMD should be "inherit", for the other mTHP sizes it >>>>> should be "never". >>>>> >>>>> So we should fail if: >>>>> * Setting top-level to "force" when any non-PMD size is "inherit" >>>>> * Setting "inherit" of a non-PMD size when the top-level is force >>>> >>>> IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not >>>> fail. >>> >>> If force does not apply to an mTHP size, it should fail if it would get >>> inherited. Until it applies and we enable it. >>> >>> I'm still confused about all the toggles here, so could be I am missing >>> something. >> >> Yes, this is a little messy:( >> >> After thinking more, considering that 'force' is used to override the tmpfs >> mount option, and 'inherit' will inherit the global setting. Your suggestion >> will make the logic eary to understand (though it is valid for anon shmem mTHP >> allocations, which are not part of this scenario), Ryan, what do you think? > > I was thinking that ever returning an error code when trying to set "force" for > /mm/transparent_hugepage/shmem_enabled would be weird behavior and to be avoided > if possible. > > But as always, David makes a clear and compelling case. I'm happy to go with his > suggestion. I'm afraid "as always" is far from true :D > > I wonder if we should do the same for "deny"? The argument I previously made was > that non-PMD-size mTHP is always effectively denied while tmpfs doesn't support > mTHP, so we can allow "deny" and for now it would just be an alias for "never". > The semantics don't change when you introduce tmpfs support. > > Personally I lean towards allowing "deny" from day 1. David? If it has clear semantics that can be fulfilled as of today, sure, we can have it.