05158082f6
Backport a preliminary version of Yu Zhao's multi-generational LRU, for improved memory management. Refresh the patches while at it. Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
162 lines
6.3 KiB
Diff
162 lines
6.3 KiB
Diff
From f59c618ed70a1e48accc4cad91a200966f2569c9 Mon Sep 17 00:00:00 2001
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From: Yu Zhao <yuzhao@google.com>
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Date: Tue, 2 Feb 2021 01:27:45 -0700
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Subject: [PATCH 10/10] mm: multigenerational lru: documentation
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Add Documentation/vm/multigen_lru.rst.
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Signed-off-by: Yu Zhao <yuzhao@google.com>
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Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
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Change-Id: I1902178bcbb5adfa0a748c4d284a6456059bdd7e
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---
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Documentation/vm/index.rst | 1 +
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Documentation/vm/multigen_lru.rst | 132 ++++++++++++++++++++++++++++++
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2 files changed, 133 insertions(+)
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create mode 100644 Documentation/vm/multigen_lru.rst
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--- a/Documentation/vm/index.rst
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+++ b/Documentation/vm/index.rst
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@@ -17,6 +17,7 @@ various features of the Linux memory man
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swap_numa
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zswap
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+ multigen_lru
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Kernel developers MM documentation
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==================================
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--- /dev/null
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+++ b/Documentation/vm/multigen_lru.rst
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@@ -0,0 +1,132 @@
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+.. SPDX-License-Identifier: GPL-2.0
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+
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+=====================
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+Multigenerational LRU
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+=====================
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+
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+Quick Start
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+===========
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+Build Configurations
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+--------------------
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+:Required: Set ``CONFIG_LRU_GEN=y``.
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+
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+:Optional: Set ``CONFIG_LRU_GEN_ENABLED=y`` to turn the feature on by
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+ default.
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+
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+Runtime Configurations
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+----------------------
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+:Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the
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+ feature was not turned on by default.
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+
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+:Optional: Write ``N`` to ``/sys/kernel/mm/lru_gen/min_ttl_ms`` to
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+ protect the working set of ``N`` milliseconds. The OOM killer is
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+ invoked if this working set cannot be kept in memory.
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+
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+:Optional: Read ``/sys/kernel/debug/lru_gen`` to confirm the feature
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+ is turned on. This file has the following output:
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+
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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 birth_time anon_size file_size
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+ ...
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+ max_gen birth_time anon_size file_size
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+
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+``min_gen`` is the oldest generation number and ``max_gen`` is the
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+youngest generation number. ``birth_time`` is in milliseconds.
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+``anon_size`` and ``file_size`` are in pages.
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+
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+Phones/Laptops/Workstations
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+---------------------------
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+No additional configurations required.
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+
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+Servers/Data Centers
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+--------------------
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+:To support more generations: Change ``CONFIG_NR_LRU_GENS`` to a
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+ larger number.
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+
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+:To support more tiers: Change ``CONFIG_TIERS_PER_GEN`` to a larger
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+ number.
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+
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+:To support full stats: Set ``CONFIG_LRU_GEN_STATS=y``.
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+
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+:Working set estimation: Write ``+ memcg_id node_id max_gen
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+ [swappiness] [use_bloom_filter]`` to ``/sys/kernel/debug/lru_gen`` to
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+ invoke the aging, which scans PTEs for accessed pages and then
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+ creates the next generation ``max_gen+1``. A swap file and a non-zero
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+ ``swappiness``, which overrides ``vm.swappiness``, are required to
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+ scan PTEs mapping anon pages. Set ``use_bloom_filter`` to 0 to
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+ override the default behavior which only scans PTE tables found
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+ populated.
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+
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+:Proactive reclaim: Write ``- memcg_id node_id min_gen [swappiness]
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+ [nr_to_reclaim]`` to ``/sys/kernel/debug/lru_gen`` to invoke the
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+ eviction, which evicts generations less than or equal to ``min_gen``.
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+ ``min_gen`` should be less than ``max_gen-1`` as ``max_gen`` and
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+ ``max_gen-1`` are not fully aged and therefore cannot be evicted.
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+ Use ``nr_to_reclaim`` to limit the number of pages to evict. Multiple
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+ command lines are supported, so does concatenation with delimiters
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+ ``,`` and ``;``.
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+
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+Framework
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+=========
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+For each ``lruvec``, evictable pages are divided into multiple
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+generations. The youngest generation number is stored in
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+``lrugen->max_seq`` for both anon and file types as they are aged on
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+an equal footing. The oldest generation numbers are stored in
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+``lrugen->min_seq[]`` separately for anon and file types as clean
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+file pages can be evicted regardless of swap and writeback
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+constraints. These three variables are monotonically increasing.
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+Generation numbers are truncated into
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+``order_base_2(CONFIG_NR_LRU_GENS+1)`` bits in order to fit into
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+``page->flags``. The sliding window technique is used to prevent
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+truncated generation numbers from overlapping. Each truncated
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+generation number is an index to an array of per-type and per-zone
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+lists ``lrugen->lists``.
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+
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+Each generation is divided into multiple tiers. Tiers represent
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+different ranges of numbers of accesses from file descriptors only.
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+Pages accessed ``N`` times via file descriptors belong to tier
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+``order_base_2(N)``. Each generation contains at most
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+``CONFIG_TIERS_PER_GEN`` tiers, and they require additional
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+``CONFIG_TIERS_PER_GEN-2`` bits in ``page->flags``. In contrast to
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+moving between generations which requires list operations, moving
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+between tiers only involves operations on ``page->flags`` and
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+therefore has a negligible cost. A feedback loop modeled after the PID
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+controller monitors refaulted % across all tiers and decides when to
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+protect pages from which tiers.
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+
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+The framework comprises two conceptually independent components: the
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+aging and the eviction, which can be invoked separately from user
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+space for the purpose of working set estimation and proactive reclaim.
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+
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+Aging
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+-----
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+The aging produces young generations. Given an ``lruvec``, the aging
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+traverses ``lruvec_memcg()->mm_list`` and calls ``walk_page_range()``
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+to scan PTEs for accessed pages (a ``mm_struct`` list is maintained
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+for each ``memcg``). Upon finding one, the aging updates its
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+generation number to ``max_seq`` (modulo ``CONFIG_NR_LRU_GENS``).
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+After each round of traversal, the aging increments ``max_seq``. The
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+aging is due when ``min_seq[]`` reaches ``max_seq-1``.
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+
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+Eviction
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+--------
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+The eviction consumes old generations. Given an ``lruvec``, the
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+eviction scans pages on the per-zone lists indexed by anon and file
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+``min_seq[]`` (modulo ``CONFIG_NR_LRU_GENS``). It first tries to
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+select a type based on the values of ``min_seq[]``. If they are
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+equal, it selects the type that has a lower refaulted %. The eviction
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+sorts a page according to its updated generation number if the aging
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+has found this page accessed. It also moves a page to the next
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+generation if this page is from an upper tier that has a higher
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+refaulted % than the base tier. The eviction increments ``min_seq[]``
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+of a selected type when it finds all the per-zone lists indexed by
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+``min_seq[]`` of this selected type are empty.
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+
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+To-do List
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+==========
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+KVM Optimization
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+----------------
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+Support shadow page table walk.
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