323 lines
14 KiB
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323 lines
14 KiB
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Date: Tue, 13 Apr 2021 00:56:33 -0600
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In-Reply-To: <20210413065633.2782273-1-yuzhao@google.com>
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Message-Id: <20210413065633.2782273-17-yuzhao@google.com>
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Subject: [PATCH v2 16/16] mm: multigenerational lru: documentation
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From: Yu Zhao <yuzhao@google.com>
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To: linux-mm@kvack.org
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Cc: Alex Shi <alexs@kernel.org>, Andi Kleen <ak@linux.intel.com>,
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Andrew Morton <akpm@linux-foundation.org>,
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Benjamin Manes <ben.manes@gmail.com>,
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Dave Chinner <david@fromorbit.com>,
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Dave Hansen <dave.hansen@linux.intel.com>,
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Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
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Johannes Weiner <hannes@cmpxchg.org>,
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Jonathan Corbet <corbet@lwn.net>,
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Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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Matthew Wilcox <willy@infradead.org>,
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Mel Gorman <mgorman@suse.de>,
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Miaohe Lin <linmiaohe@huawei.com>,
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Michael Larabel <michael@michaellarabel.com>,
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Michal Hocko <mhocko@suse.com>,
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Michel Lespinasse <michel@lespinasse.org>,
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Rik van Riel <riel@surriel.com>,
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Roman Gushchin <guro@fb.com>,
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Rong Chen <rong.a.chen@intel.com>,
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SeongJae Park <sjpark@amazon.de>,
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Tim Chen <tim.c.chen@linux.intel.com>,
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Vlastimil Babka <vbabka@suse.cz>,
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Yang Shi <shy828301@gmail.com>,
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Ying Huang <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
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linux-kernel@vger.kernel.org, lkp@lists.01.org,
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page-reclaim@google.com, Yu Zhao <yuzhao@google.com>
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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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---
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Documentation/vm/index.rst | 1 +
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Documentation/vm/multigen_lru.rst | 192 ++++++++++++++++++++++++++++++
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2 files changed, 193 insertions(+)
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create mode 100644 Documentation/vm/multigen_lru.rst
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diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst
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index eff5fbd492d0..c353b3f55924 100644
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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 management
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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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diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst
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new file mode 100644
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index 000000000000..cf772aeca317
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--- /dev/null
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+++ b/Documentation/vm/multigen_lru.rst
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@@ -0,0 +1,192 @@
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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 Options
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+-------------
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+:Required: Set ``CONFIG_LRU_GEN=y``.
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+
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+:Optional: Change ``CONFIG_NR_LRU_GENS`` to a number ``X`` to support
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+ a maximum of ``X`` generations.
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+
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+:Optional: Change ``CONFIG_TIERS_PER_GEN`` to a number ``Y`` to support
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+ a maximum of ``Y`` tiers per generation.
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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 Options
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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: Change ``/sys/kernel/mm/lru_gen/spread`` to a number ``N``
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+ to spread pages out across ``N+1`` generations. ``N`` should be less
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+ than ``X``. Larger values make the background aging more aggressive.
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+
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+:Optional: Read ``/sys/kernel/debug/lru_gen`` to verify the feature.
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+ 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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+Given a memcg and a node, ``min_gen`` is the oldest generation
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+(number) and ``max_gen`` is the youngest. Birth time is in
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+milliseconds. The sizes of anon and file types are in pages.
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+
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+Recipes
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+-------
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+:Android on ARMv8.1+: ``X=4``, ``N=0``
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+
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+:Android on pre-ARMv8.1 CPUs: Not recommended due to the lack of
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+ ``ARM64_HW_AFDBM``
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+
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+:Laptops running Chrome on x86_64: ``X=7``, ``N=2``
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+
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+:Working set estimation: Write ``+ memcg_id node_id gen [swappiness]``
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+ to ``/sys/kernel/debug/lru_gen`` to account referenced pages to
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+ generation ``max_gen`` and create the next generation ``max_gen+1``.
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+ ``gen`` should be equal to ``max_gen``. A swap file and a non-zero
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+ ``swappiness`` are required to scan anon type. If swapping is not
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+ desired, set ``vm.swappiness`` to ``0``.
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+
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+:Proactive reclaim: Write ``- memcg_id node_id gen [swappiness]
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+ [nr_to_reclaim]`` to ``/sys/kernel/debug/lru_gen`` to evict
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+ generations less than or equal to ``gen``. ``gen`` should be less
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+ than ``max_gen-1`` as ``max_gen`` and ``max_gen-1`` are active
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+ generations and therefore protected from the eviction. Use
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+ ``nr_to_reclaim`` to limit the number of pages to be evicted.
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+ Multiple command lines are supported, so does concatenation with
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+ delimiters ``,`` 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 ``max_seq``
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+for both anon and file types as they are aged on an equal footing. The
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+oldest generation numbers are stored in ``min_seq[2]`` separately for
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+anon and file types as clean file pages can be evicted regardless of
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+swap and write-back constraints. 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. Evictable pages are added to the per-zone lists indexed by
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+``max_seq`` or ``min_seq[2]`` (modulo ``CONFIG_NR_LRU_GENS``),
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+depending on whether they are being faulted in.
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+
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+Each generation is then divided into multiple tiers. Tiers represent
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+levels of usage from file descriptors only. Pages accessed N times via
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+file descriptors belong to tier order_base_2(N). In contrast to moving
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+across generations which requires the lru lock, moving across tiers
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+only involves an atomic operation on ``page->flags`` and therefore has
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+a negligible cost.
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+
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+The workflow comprises two conceptually independent functions: the
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+aging and the eviction.
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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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+scans page tables for referenced pages of this ``lruvec``. Upon
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+finding one, the aging updates its generation number to ``max_seq``.
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+After each round of scan, the aging increments ``max_seq``.
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+
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+The aging maintains either a system-wide ``mm_struct`` list or
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+per-memcg ``mm_struct`` lists, and it only scans page tables of
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+processes that have been scheduled since the last scan. Since scans
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+are differential with respect to referenced pages, the cost is roughly
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+proportional to their number.
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+
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+The aging is due when both of ``min_seq[2]`` reaches ``max_seq-1``,
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+assuming both anon and file types are reclaimable.
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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 the pages on the per-zone lists indexed by either of
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+``min_seq[2]``. It first tries to select a type based on the values of
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+``min_seq[2]``. When anon and file types are both available from the
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+same generation, it selects the one that has a lower refault rate.
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+
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+During a scan, the eviction sorts pages according to their generation
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+numbers, if the aging has found them referenced. It also moves pages
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+from the tiers that have higher refault rates than tier 0 to the next
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+generation.
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+
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+When it finds all the per-zone lists of a selected type are empty, the
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+eviction increments ``min_seq[2]`` indexed by this selected type.
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+
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+Rationale
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+=========
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+Limitations of Current Implementation
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+-------------------------------------
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+Notion of Active/Inactive
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+~~~~~~~~~~~~~~~~~~~~~~~~~
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+For servers equipped with hundreds of gigabytes of memory, the
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+granularity of the active/inactive is too coarse to be useful for job
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+scheduling. False active/inactive rates are relatively high, and thus
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+the assumed savings may not materialize.
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+
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+For phones and laptops, executable pages are frequently evicted
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+despite the fact that there are many less recently used anon pages.
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+Major faults on executable pages cause ``janks`` (slow UI renderings)
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+and negatively impact user experience.
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+
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+For ``lruvec``\s from different memcgs or nodes, comparisons are
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+impossible due to the lack of a common frame of reference.
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+
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+Incremental Scans via ``rmap``
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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+Each incremental scan picks up at where the last scan left off and
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+stops after it has found a handful of unreferenced pages. For
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+workloads using a large amount of anon memory, incremental scans lose
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+the advantage under sustained memory pressure due to high ratios of
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+the number of scanned pages to the number of reclaimed pages. On top
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+of that, the ``rmap`` has poor memory locality due to its complex data
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+structures. The combined effects typically result in a high amount of
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+CPU usage in the reclaim path.
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+
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+Benefits of Multigenerational LRU
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+---------------------------------
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+Notion of Generation Numbers
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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+The notion of generation numbers introduces a quantitative approach to
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+memory overcommit. A larger number of pages can be spread out across
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+configurable generations, and thus they have relatively low false
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+active/inactive rates. Each generation includes all pages that have
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+been referenced since the last generation.
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+
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+Given an ``lruvec``, scans and the selections between anon and file
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+types are all based on generation numbers, which are simple and yet
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+effective. For different ``lruvec``\s, comparisons are still possible
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+based on birth times of generations.
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+
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+Differential Scans via Page Tables
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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+Each differential scan discovers all pages that have been referenced
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+since the last scan. Specifically, it walks the ``mm_struct`` list
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+associated with an ``lruvec`` to scan page tables of processes that
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+have been scheduled since the last scan. The cost of each differential
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+scan is roughly proportional to the number of referenced pages it
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+discovers. Unless address spaces are extremely sparse, page tables
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+usually have better memory locality than the ``rmap``. The end result
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+is generally a significant reduction in CPU usage, for workloads
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+using a large amount of anon memory.
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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 scanning.
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+
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+NUMA Optimization
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+-----------------
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+Support NUMA policies and per-node RSS counters.
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--
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2.31.1.295.g9ea45b61b8-goog
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