| Message ID | 20250414222456.43212-3-npache@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: introduce THP deferred setting | expand |
On 4/14/25 3:24 PM, Nico Pache wrote: > The new defer option for (m)THPs allows for a more conservative > approach to (m)THPs. Document its usage in the transhuge admin-guide. > > Signed-off-by: Nico Pache <npache@redhat.com> > --- > Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------ > 1 file changed, 23 insertions(+), 8 deletions(-) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > index f0d4e78cedaa..d3f072bdd932 100644 > --- a/Documentation/admin-guide/mm/transhuge.rst > +++ b/Documentation/admin-guide/mm/transhuge.rst > @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide, application > may end up allocating more memory resources. An application may mmap a > large region but only touch 1 byte of it, in that case a 2M page might > be allocated instead of a 4k page for no good. This is why it's > -possible to disable hugepages system-wide and to only have them inside > -MADV_HUGEPAGE madvise regions. > +possible to disable hugepages system-wide, only have them inside > +MADV_HUGEPAGE madvise regions, or defer them away from the page fault > +handler to khugepaged. > > Embedded systems should enable hugepages only inside madvise regions > to eliminate any risk of wasting any precious byte of memory and to > @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages and that don't > risk to lose memory by using hugepages, should use > madvise(MADV_HUGEPAGE) on their critical mmapped regions. > > +Applications that would like to benefit from THPs but would still like a > +more memory conservative approach can choose 'defer'. This avoids > +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE. > +Khugepaged will then scan the mappings for potential collapses into (m)THP > +pages. Admins using this the 'defer' setting should consider > +tweaking khugepaged/max_ptes_none. The current default of 511 may > +aggressively collapse your PTEs into PMDs. Lower this value to conserve > +more memory (ie. max_ptes_none=64). i.e., > + > .. _thp_sysfs: > > sysfs > @@ -109,11 +119,14 @@ Global THP controls > > Transparent Hugepage Support for anonymous memory can be entirely disabled > (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE > -regions (to avoid the risk of consuming more memory resources) or enabled > -system wide. This can be achieved per-supported-THP-size with one of:: > +regions (to avoid the risk of consuming more memory resources), defered to deferred > +khugepaged, or enabled system wide. > + > +This can be achieved per-supported-THP-size with one of:: > > echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > + echo defer >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled > > where <size> is the hugepage size being addressed, the available sizes > @@ -136,6 +149,7 @@ The top-level setting (for use with "inherit") can be set by issuing > one of the following commands:: > > echo always >/sys/kernel/mm/transparent_hugepage/enabled > + echo defer >/sys/kernel/mm/transparent_hugepage/enabled > echo madvise >/sys/kernel/mm/transparent_hugepage/enabled > echo never >/sys/kernel/mm/transparent_hugepage/enabled > > @@ -281,7 +295,8 @@ of small pages into one large page:: > A higher value leads to use additional memory for programs. > A lower value leads to gain less thp performance. Value of > max_ptes_none can waste cpu time very little, you can > -ignore it. > +ignore it. Consider lowering this value when using > +``transparent_hugepage=defer`` > > ``max_ptes_swap`` specifies how many pages can be brought in from > swap when collapsing a group of pages into a transparent huge page:: > @@ -306,14 +321,14 @@ Boot parameters > > You can change the sysfs boot time default for the top-level "enabled" > control by passing the parameter ``transparent_hugepage=always`` or > -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the > -kernel command line. > +``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or > +``transparent_hugepage=never`` to the kernel command line. > > Alternatively, each supported anonymous THP size can be controlled by > passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``, > where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and > supported anonymous THP) and ``<state>`` is one of ``always``, ``madvise``, > -``never`` or ``inherit``. > +``defer``, ``never`` or ``inherit``. > > For example, the following will set 16K, 32K, 64K THP to ``always``, > set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M
diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst index f0d4e78cedaa..d3f072bdd932 100644 --- a/Documentation/admin-guide/mm/transhuge.rst +++ b/Documentation/admin-guide/mm/transhuge.rst @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide, application may end up allocating more memory resources. An application may mmap a large region but only touch 1 byte of it, in that case a 2M page might be allocated instead of a 4k page for no good. This is why it's -possible to disable hugepages system-wide and to only have them inside -MADV_HUGEPAGE madvise regions. +possible to disable hugepages system-wide, only have them inside +MADV_HUGEPAGE madvise regions, or defer them away from the page fault +handler to khugepaged. Embedded systems should enable hugepages only inside madvise regions to eliminate any risk of wasting any precious byte of memory and to @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages and that don't risk to lose memory by using hugepages, should use madvise(MADV_HUGEPAGE) on their critical mmapped regions. +Applications that would like to benefit from THPs but would still like a +more memory conservative approach can choose 'defer'. This avoids +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE. +Khugepaged will then scan the mappings for potential collapses into (m)THP +pages. Admins using this the 'defer' setting should consider +tweaking khugepaged/max_ptes_none. The current default of 511 may +aggressively collapse your PTEs into PMDs. Lower this value to conserve +more memory (ie. max_ptes_none=64). + .. _thp_sysfs: sysfs @@ -109,11 +119,14 @@ Global THP controls Transparent Hugepage Support for anonymous memory can be entirely disabled (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE -regions (to avoid the risk of consuming more memory resources) or enabled -system wide. This can be achieved per-supported-THP-size with one of:: +regions (to avoid the risk of consuming more memory resources), defered to +khugepaged, or enabled system wide. + +This can be achieved per-supported-THP-size with one of:: echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled + echo defer >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled where <size> is the hugepage size being addressed, the available sizes @@ -136,6 +149,7 @@ The top-level setting (for use with "inherit") can be set by issuing one of the following commands:: echo always >/sys/kernel/mm/transparent_hugepage/enabled + echo defer >/sys/kernel/mm/transparent_hugepage/enabled echo madvise >/sys/kernel/mm/transparent_hugepage/enabled echo never >/sys/kernel/mm/transparent_hugepage/enabled @@ -281,7 +295,8 @@ of small pages into one large page:: A higher value leads to use additional memory for programs. A lower value leads to gain less thp performance. Value of max_ptes_none can waste cpu time very little, you can -ignore it. +ignore it. Consider lowering this value when using +``transparent_hugepage=defer`` ``max_ptes_swap`` specifies how many pages can be brought in from swap when collapsing a group of pages into a transparent huge page:: @@ -306,14 +321,14 @@ Boot parameters You can change the sysfs boot time default for the top-level "enabled" control by passing the parameter ``transparent_hugepage=always`` or -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the -kernel command line. +``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or +``transparent_hugepage=never`` to the kernel command line. Alternatively, each supported anonymous THP size can be controlled by passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``, where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and supported anonymous THP) and ``<state>`` is one of ``always``, ``madvise``, -``never`` or ``inherit``. +``defer``, ``never`` or ``inherit``. For example, the following will set 16K, 32K, 64K THP to ``always``, set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M
The new defer option for (m)THPs allows for a more conservative approach to (m)THPs. Document its usage in the transhuge admin-guide. Signed-off-by: Nico Pache <npache@redhat.com> --- Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------ 1 file changed, 23 insertions(+), 8 deletions(-)