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