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authorT.J. Alumbaugh <talumbau@google.com>2023-01-18 03:18:24 +0300
committerAndrew Morton <akpm@linux-foundation.org>2023-02-03 09:33:27 +0300
commit36c7b4db7c942ae9e1b111f0c6b468c8b2e33842 (patch)
treedb10757d8ae91e1ee93b3c2b272adfb934c2fa66 /Documentation/mm
parentccbbbb85945d8f0255aa9dbc1b617017e2294f2c (diff)
downloadlinux-36c7b4db7c942ae9e1b111f0c6b468c8b2e33842.tar.xz
mm: multi-gen LRU: section for memcg LRU
Move memcg LRU code into a dedicated section. Improve the design doc to outline its architecture. Link: https://lkml.kernel.org/r/20230118001827.1040870-5-talumbau@google.com Signed-off-by: T.J. Alumbaugh <talumbau@google.com> Cc: Yu Zhao <yuzhao@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'Documentation/mm')
-rw-r--r--Documentation/mm/multigen_lru.rst33
1 files changed, 32 insertions, 1 deletions
diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index 770b5d539856..5f1f6ecbb79b 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -186,9 +186,40 @@ is false positive, the cost is an additional scan of a range of PTEs,
which may yield hot pages anyway. Parameters of the filter itself can
control the false positive rate in the limit.
+Memcg LRU
+---------
+An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
+since each node and memcg combination has an LRU of folios (see
+``mem_cgroup_lruvec()``). Its goal is to improve the scalability of
+global reclaim, which is critical to system-wide memory overcommit in
+data centers. Note that memcg LRU only applies to global reclaim.
+
+The basic structure of an memcg LRU can be understood by an analogy to
+the active/inactive LRU (of folios):
+
+1. It has the young and the old (generations), i.e., the counterparts
+ to the active and the inactive;
+2. The increment of ``max_seq`` triggers promotion, i.e., the
+ counterpart to activation;
+3. Other events trigger similar operations, e.g., offlining an memcg
+ triggers demotion, i.e., the counterpart to deactivation.
+
+In terms of global reclaim, it has two distinct features:
+
+1. Sharding, which allows each thread to start at a random memcg (in
+ the old generation) and improves parallelism;
+2. Eventual fairness, which allows direct reclaim to bail out at will
+ and reduces latency without affecting fairness over some time.
+
+In terms of traversing memcgs during global reclaim, it improves the
+best-case complexity from O(n) to O(1) and does not affect the
+worst-case complexity O(n). Therefore, on average, it has a sublinear
+complexity.
+
Summary
-------
-The multi-gen LRU can be disassembled into the following parts:
+The multi-gen LRU (of folios) can be disassembled into the following
+parts:
* Generations
* Rmap walks