| Message ID | 20220918080010.2920238-14-yuzhao@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Multi-Gen LRU Framework | expand |
On Sun, Sep 18, 2022 at 02:00:10AM -0600, Yu Zhao wrote: > Add an admin guide. > > Signed-off-by: Yu Zhao <yuzhao@google.com> > Acked-by: Brian Geffon <bgeffon@google.com> > Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org> > Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name> > Acked-by: Steven Barrett <steven@liquorix.net> > Acked-by: Suleiman Souhlal <suleiman@google.com> > Tested-by: Daniel Byrne <djbyrne@mtu.edu> > Tested-by: Donald Carr <d@chaos-reins.com> > Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> > Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru> > Tested-by: Shuang Zhai <szhai2@cs.rochester.edu> > Tested-by: Sofia Trinh <sofia.trinh@edi.works> > Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com> > --- > Documentation/admin-guide/mm/index.rst | 1 + > Documentation/admin-guide/mm/multigen_lru.rst | 162 ++++++++++++++++++ > mm/Kconfig | 3 +- > mm/vmscan.c | 4 + > 4 files changed, 169 insertions(+), 1 deletion(-) > create mode 100644 Documentation/admin-guide/mm/multigen_lru.rst I have a feeling I already did this, but whatever. Acked-by: Mike Rapoport <rppt@linux.ibm.com> > > diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst > index 1bd11118dfb1..d1064e0ba34a 100644 > --- a/Documentation/admin-guide/mm/index.rst > +++ b/Documentation/admin-guide/mm/index.rst > @@ -32,6 +32,7 @@ the Linux memory management. > idle_page_tracking > ksm > memory-hotplug > + multigen_lru > nommu-mmap > numa_memory_policy > numaperf > diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst > new file mode 100644 > index 000000000000..33e068830497 > --- /dev/null > +++ b/Documentation/admin-guide/mm/multigen_lru.rst > @@ -0,0 +1,162 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +============= > +Multi-Gen LRU > +============= > +The multi-gen LRU is an alternative LRU implementation that optimizes > +page reclaim and improves performance under memory pressure. Page > +reclaim decides the kernel's caching policy and ability to overcommit > +memory. It directly impacts the kswapd CPU usage and RAM efficiency. > + > +Quick start > +=========== > +Build the kernel with the following configurations. > + > +* ``CONFIG_LRU_GEN=y`` > +* ``CONFIG_LRU_GEN_ENABLED=y`` > + > +All set! > + > +Runtime options > +=============== > +``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the > +following subsections. > + > +Kill switch > +----------- > +``enabled`` accepts different values to enable or disable the > +following components. Its default value depends on > +``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled > +unless some of them have unforeseen side effects. Writing to > +``enabled`` has no effect when a component is not supported by the > +hardware, and valid values will be accepted even when the main switch > +is off. > + > +====== =============================================================== > +Values Components > +====== =============================================================== > +0x0001 The main switch for the multi-gen LRU. > +0x0002 Clearing the accessed bit in leaf page table entries in large > + batches, when MMU sets it (e.g., on x86). This behavior can > + theoretically worsen lock contention (mmap_lock). If it is > + disabled, the multi-gen LRU will suffer a minor performance > + degradation for workloads that contiguously map hot pages, > + whose accessed bits can be otherwise cleared by fewer larger > + batches. > +0x0004 Clearing the accessed bit in non-leaf page table entries as > + well, when MMU sets it (e.g., on x86). This behavior was not > + verified on x86 varieties other than Intel and AMD. If it is > + disabled, the multi-gen LRU will suffer a negligible > + performance degradation. > +[yYnN] Apply to all the components above. > +====== =============================================================== > + > +E.g., > +:: > + > + echo y >/sys/kernel/mm/lru_gen/enabled > + cat /sys/kernel/mm/lru_gen/enabled > + 0x0007 > + echo 5 >/sys/kernel/mm/lru_gen/enabled > + cat /sys/kernel/mm/lru_gen/enabled > + 0x0005 > + > +Thrashing prevention > +-------------------- > +Personal computers are more sensitive to thrashing because it can > +cause janks (lags when rendering UI) and negatively impact user > +experience. The multi-gen LRU offers thrashing prevention to the > +majority of laptop and desktop users who do not have ``oomd``. > + > +Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of > +``N`` milliseconds from getting evicted. The OOM killer is triggered > +if this working set cannot be kept in memory. In other words, this > +option works as an adjustable pressure relief valve, and when open, it > +terminates applications that are hopefully not being used. > + > +Based on the average human detectable lag (~100ms), ``N=1000`` usually > +eliminates intolerable janks due to thrashing. Larger values like > +``N=3000`` make janks less noticeable at the risk of premature OOM > +kills. > + > +The default value ``0`` means disabled. > + > +Experimental features > +===================== > +``/sys/kernel/debug/lru_gen`` accepts commands described in the > +following subsections. Multiple command lines are supported, so does > +concatenation with delimiters ``,`` and ``;``. > + > +``/sys/kernel/debug/lru_gen_full`` provides additional stats for > +debugging. ``CONFIG_LRU_GEN_STATS=y`` keeps historical stats from > +evicted generations in this file. > + > +Working set estimation > +---------------------- > +Working set estimation measures how much memory an application needs > +in a given time interval, and it is usually done with little impact on > +the performance of the application. E.g., data centers want to > +optimize job scheduling (bin packing) to improve memory utilizations. > +When a new job comes in, the job scheduler needs to find out whether > +each server it manages can allocate a certain amount of memory for > +this new job before it can pick a candidate. To do so, the job > +scheduler needs to estimate the working sets of the existing jobs. > + > +When it is read, ``lru_gen`` returns a histogram of numbers of pages > +accessed over different time intervals for each memcg and node. > +``MAX_NR_GENS`` decides the number of bins for each histogram. The > +histograms are noncumulative. > +:: > + > + memcg memcg_id memcg_path > + node node_id > + min_gen_nr age_in_ms nr_anon_pages nr_file_pages > + ... > + max_gen_nr age_in_ms nr_anon_pages nr_file_pages > + > +Each bin contains an estimated number of pages that have been accessed > +within ``age_in_ms``. E.g., ``min_gen_nr`` contains the coldest pages > +and ``max_gen_nr`` contains the hottest pages, since ``age_in_ms`` of > +the former is the largest and that of the latter is the smallest. > + > +Users can write the following command to ``lru_gen`` to create a new > +generation ``max_gen_nr+1``: > + > + ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]`` > + > +``can_swap`` defaults to the swap setting and, if it is set to ``1``, > +it forces the scan of anon pages when swap is off, and vice versa. > +``force_scan`` defaults to ``1`` and, if it is set to ``0``, it > +employs heuristics to reduce the overhead, which is likely to reduce > +the coverage as well. > + > +A typical use case is that a job scheduler runs this command at a > +certain time interval to create new generations, and it ranks the > +servers it manages based on the sizes of their cold pages defined by > +this time interval. > + > +Proactive reclaim > +----------------- > +Proactive reclaim induces page reclaim when there is no memory > +pressure. It usually targets cold pages only. E.g., when a new job > +comes in, the job scheduler wants to proactively reclaim cold pages on > +the server it selected, to improve the chance of successfully landing > +this new job. > + > +Users can write the following command to ``lru_gen`` to evict > +generations less than or equal to ``min_gen_nr``. > + > + ``- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]`` > + > +``min_gen_nr`` should be less than ``max_gen_nr-1``, since > +``max_gen_nr`` and ``max_gen_nr-1`` are not fully aged (equivalent to > +the active list) and therefore cannot be evicted. ``swappiness`` > +overrides the default value in ``/proc/sys/vm/swappiness``. > +``nr_to_reclaim`` limits the number of pages to evict. > + > +A typical use case is that a job scheduler runs this command before it > +tries to land a new job on a server. If it fails to materialize enough > +cold pages because of the overestimation, it retries on the next > +server according to the ranking result obtained from the working set > +estimation step. This less forceful approach limits the impacts on the > +existing jobs. > diff --git a/mm/Kconfig b/mm/Kconfig > index ab6ef5115eb8..ceec438c0741 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1125,7 +1125,8 @@ config LRU_GEN > # make sure folio->flags has enough spare bits > depends on 64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP > help > - A high performance LRU implementation to overcommit memory. > + A high performance LRU implementation to overcommit memory. See > + Documentation/admin-guide/mm/multigen_lru.rst for details. > > config LRU_GEN_ENABLED > bool "Enable by default" > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 7657d54c9c42..1456f133f256 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -5310,6 +5310,7 @@ static ssize_t show_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, c > return sprintf(buf, "%u\n", jiffies_to_msecs(READ_ONCE(lru_gen_min_ttl))); > } > > +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ > static ssize_t store_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, > const char *buf, size_t len) > { > @@ -5343,6 +5344,7 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c > return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps); > } > > +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ > static ssize_t store_enabled(struct kobject *kobj, struct kobj_attribute *attr, > const char *buf, size_t len) > { > @@ -5490,6 +5492,7 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec, > seq_putc(m, '\n'); > } > > +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ > static int lru_gen_seq_show(struct seq_file *m, void *v) > { > unsigned long seq; > @@ -5648,6 +5651,7 @@ static int run_cmd(char cmd, int memcg_id, int nid, unsigned long seq, > return err; > } > > +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ > static ssize_t lru_gen_seq_write(struct file *file, const char __user *src, > size_t len, loff_t *pos) > { > -- > 2.37.3.968.ga6b4b080e4-goog >
diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst index 1bd11118dfb1..d1064e0ba34a 100644 --- a/Documentation/admin-guide/mm/index.rst +++ b/Documentation/admin-guide/mm/index.rst @@ -32,6 +32,7 @@ the Linux memory management. idle_page_tracking ksm memory-hotplug + multigen_lru nommu-mmap numa_memory_policy numaperf diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst new file mode 100644 index 000000000000..33e068830497 --- /dev/null +++ b/Documentation/admin-guide/mm/multigen_lru.rst @@ -0,0 +1,162 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +Multi-Gen LRU +============= +The multi-gen LRU is an alternative LRU implementation that optimizes +page reclaim and improves performance under memory pressure. Page +reclaim decides the kernel's caching policy and ability to overcommit +memory. It directly impacts the kswapd CPU usage and RAM efficiency. + +Quick start +=========== +Build the kernel with the following configurations. + +* ``CONFIG_LRU_GEN=y`` +* ``CONFIG_LRU_GEN_ENABLED=y`` + +All set! + +Runtime options +=============== +``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the +following subsections. + +Kill switch +----------- +``enabled`` accepts different values to enable or disable the +following components. Its default value depends on +``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled +unless some of them have unforeseen side effects. Writing to +``enabled`` has no effect when a component is not supported by the +hardware, and valid values will be accepted even when the main switch +is off. + +====== =============================================================== +Values Components +====== =============================================================== +0x0001 The main switch for the multi-gen LRU. +0x0002 Clearing the accessed bit in leaf page table entries in large + batches, when MMU sets it (e.g., on x86). This behavior can + theoretically worsen lock contention (mmap_lock). If it is + disabled, the multi-gen LRU will suffer a minor performance + degradation for workloads that contiguously map hot pages, + whose accessed bits can be otherwise cleared by fewer larger + batches. +0x0004 Clearing the accessed bit in non-leaf page table entries as + well, when MMU sets it (e.g., on x86). This behavior was not + verified on x86 varieties other than Intel and AMD. If it is + disabled, the multi-gen LRU will suffer a negligible + performance degradation. +[yYnN] Apply to all the components above. +====== =============================================================== + +E.g., +:: + + echo y >/sys/kernel/mm/lru_gen/enabled + cat /sys/kernel/mm/lru_gen/enabled + 0x0007 + echo 5 >/sys/kernel/mm/lru_gen/enabled + cat /sys/kernel/mm/lru_gen/enabled + 0x0005 + +Thrashing prevention +-------------------- +Personal computers are more sensitive to thrashing because it can +cause janks (lags when rendering UI) and negatively impact user +experience. The multi-gen LRU offers thrashing prevention to the +majority of laptop and desktop users who do not have ``oomd``. + +Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of +``N`` milliseconds from getting evicted. The OOM killer is triggered +if this working set cannot be kept in memory. In other words, this +option works as an adjustable pressure relief valve, and when open, it +terminates applications that are hopefully not being used. + +Based on the average human detectable lag (~100ms), ``N=1000`` usually +eliminates intolerable janks due to thrashing. Larger values like +``N=3000`` make janks less noticeable at the risk of premature OOM +kills. + +The default value ``0`` means disabled. + +Experimental features +===================== +``/sys/kernel/debug/lru_gen`` accepts commands described in the +following subsections. Multiple command lines are supported, so does +concatenation with delimiters ``,`` and ``;``. + +``/sys/kernel/debug/lru_gen_full`` provides additional stats for +debugging. ``CONFIG_LRU_GEN_STATS=y`` keeps historical stats from +evicted generations in this file. + +Working set estimation +---------------------- +Working set estimation measures how much memory an application needs +in a given time interval, and it is usually done with little impact on +the performance of the application. E.g., data centers want to +optimize job scheduling (bin packing) to improve memory utilizations. +When a new job comes in, the job scheduler needs to find out whether +each server it manages can allocate a certain amount of memory for +this new job before it can pick a candidate. To do so, the job +scheduler needs to estimate the working sets of the existing jobs. + +When it is read, ``lru_gen`` returns a histogram of numbers of pages +accessed over different time intervals for each memcg and node. +``MAX_NR_GENS`` decides the number of bins for each histogram. The +histograms are noncumulative. +:: + + memcg memcg_id memcg_path + node node_id + min_gen_nr age_in_ms nr_anon_pages nr_file_pages + ... + max_gen_nr age_in_ms nr_anon_pages nr_file_pages + +Each bin contains an estimated number of pages that have been accessed +within ``age_in_ms``. E.g., ``min_gen_nr`` contains the coldest pages +and ``max_gen_nr`` contains the hottest pages, since ``age_in_ms`` of +the former is the largest and that of the latter is the smallest. + +Users can write the following command to ``lru_gen`` to create a new +generation ``max_gen_nr+1``: + + ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]`` + +``can_swap`` defaults to the swap setting and, if it is set to ``1``, +it forces the scan of anon pages when swap is off, and vice versa. +``force_scan`` defaults to ``1`` and, if it is set to ``0``, it +employs heuristics to reduce the overhead, which is likely to reduce +the coverage as well. + +A typical use case is that a job scheduler runs this command at a +certain time interval to create new generations, and it ranks the +servers it manages based on the sizes of their cold pages defined by +this time interval. + +Proactive reclaim +----------------- +Proactive reclaim induces page reclaim when there is no memory +pressure. It usually targets cold pages only. E.g., when a new job +comes in, the job scheduler wants to proactively reclaim cold pages on +the server it selected, to improve the chance of successfully landing +this new job. + +Users can write the following command to ``lru_gen`` to evict +generations less than or equal to ``min_gen_nr``. + + ``- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]`` + +``min_gen_nr`` should be less than ``max_gen_nr-1``, since +``max_gen_nr`` and ``max_gen_nr-1`` are not fully aged (equivalent to +the active list) and therefore cannot be evicted. ``swappiness`` +overrides the default value in ``/proc/sys/vm/swappiness``. +``nr_to_reclaim`` limits the number of pages to evict. + +A typical use case is that a job scheduler runs this command before it +tries to land a new job on a server. If it fails to materialize enough +cold pages because of the overestimation, it retries on the next +server according to the ranking result obtained from the working set +estimation step. This less forceful approach limits the impacts on the +existing jobs. diff --git a/mm/Kconfig b/mm/Kconfig index ab6ef5115eb8..ceec438c0741 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -1125,7 +1125,8 @@ config LRU_GEN # make sure folio->flags has enough spare bits depends on 64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP help - A high performance LRU implementation to overcommit memory. + A high performance LRU implementation to overcommit memory. See + Documentation/admin-guide/mm/multigen_lru.rst for details. config LRU_GEN_ENABLED bool "Enable by default" diff --git a/mm/vmscan.c b/mm/vmscan.c index 7657d54c9c42..1456f133f256 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -5310,6 +5310,7 @@ static ssize_t show_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, c return sprintf(buf, "%u\n", jiffies_to_msecs(READ_ONCE(lru_gen_min_ttl))); } +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t store_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t len) { @@ -5343,6 +5344,7 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps); } +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t store_enabled(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t len) { @@ -5490,6 +5492,7 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec, seq_putc(m, '\n'); } +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static int lru_gen_seq_show(struct seq_file *m, void *v) { unsigned long seq; @@ -5648,6 +5651,7 @@ static int run_cmd(char cmd, int memcg_id, int nid, unsigned long seq, return err; } +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t lru_gen_seq_write(struct file *file, const char __user *src, size_t len, loff_t *pos) {