Re: [PATCH v14 13/14] mm: multi-gen LRU: admin guide

[Mike Rapoport]

On Mon, Aug 15, 2022 at 01:13:32AM -0600, Yu Zhao wrote:
> Add an admin guide.
> 
> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Acked-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> Acked-by: Jan Alexander Steffens (heftig) <heftig@xxxxxxxxxxxxx>
> Acked-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
> Acked-by: Steven Barrett <steven@xxxxxxxxxxxx>
> Acked-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Tested-by: Daniel Byrne <djbyrne@xxxxxxx>
> Tested-by: Donald Carr <d@xxxxxxxxxxxxxxx>
> Tested-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
> Tested-by: Konstantin Kharlamov <Hi-Angel@xxxxxxxxx>
> Tested-by: Shuang Zhai <szhai2@xxxxxxxxxxxxxxxx>
> Tested-by: Sofia Trinh <sofia.trinh@edi.works>
> Tested-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>

A few formatting nits below, otherwise

Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
>  Documentation/admin-guide/mm/index.rst        |   1 +
>  Documentation/admin-guide/mm/multigen_lru.rst | 156 ++++++++++++++++++
>  mm/Kconfig                                    |   3 +-
>  mm/vmscan.c                                   |   4 +
>  4 files changed, 163 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/admin-guide/mm/multigen_lru.rst
> 
> 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..6355f2b5019d
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/multigen_lru.rst
> @@ -0,0 +1,156 @@
> +.. 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 ``+ memcg_id node_id max_gen_nr
> +[can_swap [force_scan]]`` to ``lru_gen`` to create a new generation

I think this would look nicer if the command would be a literal block, say

Users can write:

	+ memcg_id node_id max_gen_nr [can swap [force_scan]]

to ``lru_gen`` ...

> +``max_gen_nr+1``. ``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 writes to ``lru_gen`` 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 ``- memcg_id node_id min_gen_nr [swappiness
> +[nr_to_reclaim]]`` to ``lru_gen`` to evict generations less than or

Same here.

> +equal to ``min_gen_nr``. Note that ``min_gen_nr`` should be less than
> +``max_gen_nr-1`` as ``max_gen_nr`` and ``max_gen_nr-1`` are not fully
> +aged 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 writes to ``lru_gen``
> +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 6c86849c4db9..96cd3ae25c6f 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1131,7 +1131,8 @@ 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 509989fb39ef..f693720047db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5288,6 +5288,7 @@ 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)
>  {
> @@ -5321,6 +5322,7 @@ 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)
>  {
> @@ -5468,6 +5470,7 @@ 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;
> @@ -5626,6 +5629,7 @@ 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)
>  {
> -- 
> 2.37.1.595.g718a3a8f04-goog
> 

-- 
Sincerely yours,
Mike.