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266 lines
10 KiB
Diff
266 lines
10 KiB
Diff
From 22199c9b30ffcc332be643577709a2af960e6786 Mon Sep 17 00:00:00 2001
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From: Yu Zhao <yuzhao@google.com>
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Date: Sun, 23 Jan 2022 16:44:43 -0700
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Subject: [PATCH 13/14] mm: multi-gen LRU: admin guide
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Add an admin guide.
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Signed-off-by: Yu Zhao <yuzhao@google.com>
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Acked-by: Brian Geffon <bgeffon@google.com>
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Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
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Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name>
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Acked-by: Steven Barrett <steven@liquorix.net>
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Acked-by: Suleiman Souhlal <suleiman@google.com>
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Tested-by: Daniel Byrne <djbyrne@mtu.edu>
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Tested-by: Donald Carr <d@chaos-reins.com>
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Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
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Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
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Tested-by: Shuang Zhai <szhai2@cs.rochester.edu>
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Tested-by: Sofia Trinh <sofia.trinh@edi.works>
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Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
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Change-Id: I1902178bcbb5adfa0a748c4d284a6456059bdd7e
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---
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Documentation/admin-guide/mm/index.rst | 1 +
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Documentation/admin-guide/mm/multigen_lru.rst | 162 ++++++++++++++++++
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mm/Kconfig | 3 +-
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mm/vmscan.c | 4 +
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4 files changed, 169 insertions(+), 1 deletion(-)
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create mode 100644 Documentation/admin-guide/mm/multigen_lru.rst
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diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
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index 1bd11118dfb1..d1064e0ba34a 100644
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--- a/Documentation/admin-guide/mm/index.rst
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+++ b/Documentation/admin-guide/mm/index.rst
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@@ -32,6 +32,7 @@ the Linux memory management.
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idle_page_tracking
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ksm
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memory-hotplug
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+ multigen_lru
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nommu-mmap
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numa_memory_policy
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numaperf
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diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst
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new file mode 100644
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index 000000000000..33e068830497
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--- /dev/null
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+++ b/Documentation/admin-guide/mm/multigen_lru.rst
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@@ -0,0 +1,162 @@
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+.. SPDX-License-Identifier: GPL-2.0
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+
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+=============
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+Multi-Gen LRU
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+=============
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+The multi-gen LRU is an alternative LRU implementation that optimizes
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+page reclaim and improves performance under memory pressure. Page
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+reclaim decides the kernel's caching policy and ability to overcommit
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+memory. It directly impacts the kswapd CPU usage and RAM efficiency.
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+
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+Quick start
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+===========
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+Build the kernel with the following configurations.
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+
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+* ``CONFIG_LRU_GEN=y``
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+* ``CONFIG_LRU_GEN_ENABLED=y``
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+
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+All set!
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+
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+Runtime options
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+===============
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+``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the
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+following subsections.
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+
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+Kill switch
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+-----------
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+``enabled`` accepts different values to enable or disable the
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+following components. Its default value depends on
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+``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled
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+unless some of them have unforeseen side effects. Writing to
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+``enabled`` has no effect when a component is not supported by the
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+hardware, and valid values will be accepted even when the main switch
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+is off.
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+
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+====== ===============================================================
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+Values Components
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+====== ===============================================================
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+0x0001 The main switch for the multi-gen LRU.
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+0x0002 Clearing the accessed bit in leaf page table entries in large
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+ batches, when MMU sets it (e.g., on x86). This behavior can
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+ theoretically worsen lock contention (mmap_lock). If it is
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+ disabled, the multi-gen LRU will suffer a minor performance
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+ degradation for workloads that contiguously map hot pages,
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+ whose accessed bits can be otherwise cleared by fewer larger
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+ batches.
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+0x0004 Clearing the accessed bit in non-leaf page table entries as
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+ well, when MMU sets it (e.g., on x86). This behavior was not
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+ verified on x86 varieties other than Intel and AMD. If it is
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+ disabled, the multi-gen LRU will suffer a negligible
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+ performance degradation.
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+[yYnN] Apply to all the components above.
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+====== ===============================================================
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+
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+E.g.,
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+::
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+
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+ echo y >/sys/kernel/mm/lru_gen/enabled
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+ cat /sys/kernel/mm/lru_gen/enabled
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+ 0x0007
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+ echo 5 >/sys/kernel/mm/lru_gen/enabled
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+ cat /sys/kernel/mm/lru_gen/enabled
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+ 0x0005
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+
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+Thrashing prevention
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+--------------------
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+Personal computers are more sensitive to thrashing because it can
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+cause janks (lags when rendering UI) and negatively impact user
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+experience. The multi-gen LRU offers thrashing prevention to the
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+majority of laptop and desktop users who do not have ``oomd``.
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+
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+Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of
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+``N`` milliseconds from getting evicted. The OOM killer is triggered
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+if this working set cannot be kept in memory. In other words, this
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+option works as an adjustable pressure relief valve, and when open, it
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+terminates applications that are hopefully not being used.
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+
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+Based on the average human detectable lag (~100ms), ``N=1000`` usually
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+eliminates intolerable janks due to thrashing. Larger values like
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+``N=3000`` make janks less noticeable at the risk of premature OOM
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+kills.
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+
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+The default value ``0`` means disabled.
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+
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+Experimental features
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+=====================
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+``/sys/kernel/debug/lru_gen`` accepts commands described in the
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+following subsections. Multiple command lines are supported, so does
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+concatenation with delimiters ``,`` and ``;``.
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+
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+``/sys/kernel/debug/lru_gen_full`` provides additional stats for
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+debugging. ``CONFIG_LRU_GEN_STATS=y`` keeps historical stats from
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+evicted generations in this file.
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+
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+Working set estimation
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+----------------------
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+Working set estimation measures how much memory an application needs
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+in a given time interval, and it is usually done with little impact on
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+the performance of the application. E.g., data centers want to
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+optimize job scheduling (bin packing) to improve memory utilizations.
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+When a new job comes in, the job scheduler needs to find out whether
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+each server it manages can allocate a certain amount of memory for
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+this new job before it can pick a candidate. To do so, the job
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+scheduler needs to estimate the working sets of the existing jobs.
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+
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+When it is read, ``lru_gen`` returns a histogram of numbers of pages
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+accessed over different time intervals for each memcg and node.
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+``MAX_NR_GENS`` decides the number of bins for each histogram. The
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+histograms are noncumulative.
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+::
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+
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+ memcg memcg_id memcg_path
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+ node node_id
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+ min_gen_nr age_in_ms nr_anon_pages nr_file_pages
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+ ...
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+ max_gen_nr age_in_ms nr_anon_pages nr_file_pages
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+
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+Each bin contains an estimated number of pages that have been accessed
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+within ``age_in_ms``. E.g., ``min_gen_nr`` contains the coldest pages
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+and ``max_gen_nr`` contains the hottest pages, since ``age_in_ms`` of
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+the former is the largest and that of the latter is the smallest.
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+
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+Users can write the following command to ``lru_gen`` to create a new
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+generation ``max_gen_nr+1``:
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+
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+ ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]``
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+
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+``can_swap`` defaults to the swap setting and, if it is set to ``1``,
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+it forces the scan of anon pages when swap is off, and vice versa.
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+``force_scan`` defaults to ``1`` and, if it is set to ``0``, it
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+employs heuristics to reduce the overhead, which is likely to reduce
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+the coverage as well.
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+
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+A typical use case is that a job scheduler runs this command at a
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+certain time interval to create new generations, and it ranks the
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+servers it manages based on the sizes of their cold pages defined by
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+this time interval.
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+
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+Proactive reclaim
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+-----------------
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+Proactive reclaim induces page reclaim when there is no memory
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+pressure. It usually targets cold pages only. E.g., when a new job
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+comes in, the job scheduler wants to proactively reclaim cold pages on
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+the server it selected, to improve the chance of successfully landing
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+this new job.
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+
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+Users can write the following command to ``lru_gen`` to evict
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+generations less than or equal to ``min_gen_nr``.
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+
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+ ``- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]``
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+
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+``min_gen_nr`` should be less than ``max_gen_nr-1``, since
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+``max_gen_nr`` and ``max_gen_nr-1`` are not fully aged (equivalent to
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+the active list) and therefore cannot be evicted. ``swappiness``
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+overrides the default value in ``/proc/sys/vm/swappiness``.
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+``nr_to_reclaim`` limits the number of pages to evict.
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+
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+A typical use case is that a job scheduler runs this command before it
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+tries to land a new job on a server. If it fails to materialize enough
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+cold pages because of the overestimation, it retries on the next
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+server according to the ranking result obtained from the working set
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+estimation step. This less forceful approach limits the impacts on the
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+existing jobs.
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diff --git a/mm/Kconfig b/mm/Kconfig
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index 6c86849c4db9..96cd3ae25c6f 100644
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--- a/mm/Kconfig
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+++ b/mm/Kconfig
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@@ -1131,7 +1131,8 @@ config LRU_GEN
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# make sure folio->flags has enough spare bits
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depends on 64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP
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help
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- A high performance LRU implementation to overcommit memory.
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+ A high performance LRU implementation to overcommit memory. See
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+ Documentation/admin-guide/mm/multigen_lru.rst for details.
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config LRU_GEN_ENABLED
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bool "Enable by default"
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diff --git a/mm/vmscan.c b/mm/vmscan.c
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index 3e7aad06299b..146a54cf1bd9 100644
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--- a/mm/vmscan.c
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+++ b/mm/vmscan.c
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@@ -5307,6 +5307,7 @@ static ssize_t show_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, c
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return sprintf(buf, "%u\n", jiffies_to_msecs(READ_ONCE(lru_gen_min_ttl)));
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}
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+/* see Documentation/admin-guide/mm/multigen_lru.rst for details */
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static ssize_t store_min_ttl(struct kobject *kobj, struct kobj_attribute *attr,
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const char *buf, size_t len)
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{
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@@ -5340,6 +5341,7 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c
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return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps);
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}
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+/* see Documentation/admin-guide/mm/multigen_lru.rst for details */
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static ssize_t store_enabled(struct kobject *kobj, struct kobj_attribute *attr,
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const char *buf, size_t len)
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{
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@@ -5487,6 +5489,7 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec,
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seq_putc(m, '\n');
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}
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+/* see Documentation/admin-guide/mm/multigen_lru.rst for details */
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static int lru_gen_seq_show(struct seq_file *m, void *v)
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{
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unsigned long seq;
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@@ -5645,6 +5648,7 @@ static int run_cmd(char cmd, int memcg_id, int nid, unsigned long seq,
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return err;
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}
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+/* see Documentation/admin-guide/mm/multigen_lru.rst for details */
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static ssize_t lru_gen_seq_write(struct file *file, const char __user *src,
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size_t len, loff_t *pos)
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{
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--
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2.17.1
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