FROMLIST: mm: multi-gen LRU: design doc
Add a design doc. Link: https://lore.kernel.org/lkml/20220309021230.721028-15-yuzhao@google.com/ 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> Bug: 227651406 Signed-off-by: Kalesh Singh <kaleshsingh@google.com> Change-Id: I1d66302e618416291ebf9647e20625fb76613c89
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Yu Zhao
Carlos Llamas
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@ -42,6 +42,7 @@ descriptions of data structures and algorithms.
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ksm
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memory-model
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mmu_notifier
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multigen_lru
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numa
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overcommit-accounting
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page_migration
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@ -0,0 +1,160 @@
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.. SPDX-License-Identifier: GPL-2.0
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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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Design overview
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===============
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Objectives
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----------
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The design objectives are:
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* Good representation of access recency
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* Try to profit from spatial locality
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* Fast paths to make obvious choices
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* Simple self-correcting heuristics
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The representation of access recency is at the core of all LRU
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implementations. In the multi-gen LRU, each generation represents a
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group of pages with similar access recency. Generations establish a
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common frame of reference and therefore help make better choices,
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e.g., between different memcgs on a computer or different computers in
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a data center (for job scheduling).
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Exploiting spatial locality improves efficiency when gathering the
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accessed bit. A rmap walk targets a single page and does not try to
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profit from discovering a young PTE. A page table walk can sweep all
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the young PTEs in an address space, but the address space can be too
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large to make a profit. The key is to optimize both methods and use
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them in combination.
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Fast paths reduce code complexity and runtime overhead. Unmapped pages
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do not require TLB flushes; clean pages do not require writeback.
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These facts are only helpful when other conditions, e.g., access
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recency, are similar. With generations as a common frame of reference,
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additional factors stand out. But obvious choices might not be good
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choices; thus self-correction is required.
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The benefits of simple self-correcting heuristics are self-evident.
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Again, with generations as a common frame of reference, this becomes
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attainable. Specifically, pages in the same generation can be
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categorized based on additional factors, and a feedback loop can
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statistically compare the refault percentages across those categories
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and infer which of them are better choices.
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Assumptions
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-----------
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The protection of hot pages and the selection of cold pages are based
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on page access channels and patterns. There are two access channels:
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* Accesses through page tables
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* Accesses through file descriptors
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The protection of the former channel is by design stronger because:
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1. The uncertainty in determining the access patterns of the former
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channel is higher due to the approximation of the accessed bit.
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2. The cost of evicting the former channel is higher due to the TLB
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flushes required and the likelihood of encountering the dirty bit.
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3. The penalty of underprotecting the former channel is higher because
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applications usually do not prepare themselves for major page
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faults like they do for blocked I/O. E.g., GUI applications
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commonly use dedicated I/O threads to avoid blocking the rendering
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threads.
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There are also two access patterns:
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* Accesses exhibiting temporal locality
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* Accesses not exhibiting temporal locality
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For the reasons listed above, the former channel is assumed to follow
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the former pattern unless ``VM_SEQ_READ`` or ``VM_RAND_READ`` is
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present, and the latter channel is assumed to follow the latter
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pattern unless outlying refaults have been observed.
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Workflow overview
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=================
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Evictable pages are divided into multiple generations for each
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``lruvec``. 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 file
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pages can be evicted regardless of swap constraints. These three
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variables are monotonically increasing.
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Generation numbers are truncated into ``order_base_2(MAX_NR_GENS+1)``
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bits in order to fit into the gen counter in ``page->flags``. Each
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truncated generation number is an index to ``lrugen->lists[]``. The
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sliding window technique is used to track at least ``MIN_NR_GENS`` and
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at most ``MAX_NR_GENS`` generations. The gen counter stores a value
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within ``[1, MAX_NR_GENS]`` while a page is on one of
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``lrugen->lists[]``; otherwise it stores zero.
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Each generation is divided into multiple tiers. Tiers represent
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different ranges of numbers of accesses through file descriptors. A
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page accessed ``N`` times through file descriptors is in tier
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``order_base_2(N)``. In contrast to moving across generations, which
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requires the LRU lock, moving across tiers only requires operations on
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``page->flags`` and therefore has a negligible cost. A feedback loop
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modeled after the PID controller monitors refaults over all the tiers
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from anon and file types and decides which tiers from which types to
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evict or protect.
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There are two conceptually independent procedures: the aging and the
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eviction. They form a closed-loop system, i.e., the page reclaim.
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Aging
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-----
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The aging produces young generations. Given an ``lruvec``, it
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increments ``max_seq`` when ``max_seq-min_seq+1`` approaches
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``MIN_NR_GENS``. The aging promotes hot pages to the youngest
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generation when it finds them accessed through page tables; the
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demotion of cold pages happens consequently when it increments
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``max_seq``. The aging uses page table walks and rmap walks to find
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young PTEs. For the former, it iterates ``lruvec_memcg()->mm_list``
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and calls ``walk_page_range()`` with each ``mm_struct`` on this list
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to scan PTEs. On finding a young PTE, it clears the accessed bit and
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updates the gen counter of the page mapped by this PTE to
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``(max_seq%MAX_NR_GENS)+1``. After each iteration of this list, it
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increments ``max_seq``. For the latter, when the eviction walks the
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rmap and finds a young PTE, the aging scans the adjacent PTEs and
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follows the same steps just described.
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Eviction
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--------
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The eviction consumes old generations. Given an ``lruvec``, it
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increments ``min_seq`` when ``lrugen->lists[]`` indexed by
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``min_seq%MAX_NR_GENS`` becomes empty. To select a type and a tier to
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evict from, it first compares ``min_seq[]`` to select the older type.
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If both types are equally old, it selects the one whose first tier has
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a lower refault percentage. The first tier contains single-use
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unmapped clean pages, which are the best bet. The eviction sorts a
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page according to the gen counter if the aging has found this page
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accessed through page tables and updated the gen counter. It also
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moves a page to the next generation, i.e., ``min_seq+1``, if this page
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was accessed multiple times through file descriptors and the feedback
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loop has detected outlying refaults from the tier this page is in. To
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do this, the feedback loop uses the first tier as the baseline, for
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the reason stated earlier.
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Summary
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-------
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The multi-gen LRU can be disassembled into the following parts:
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* Generations
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* Page table walks
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* Rmap walks
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* Bloom filters
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* The PID controller
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The aging and the eviction is a producer-consumer model; specifically,
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the latter drives the former by the sliding window over generations.
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Within the aging, rmap walks drive page table walks by inserting hot
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densely populated page tables to the Bloom filters. Within the
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eviction, the PID controller uses refaults as the feedback to select
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types to evict and tiers to protect.
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