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mm/docs: describe memory.low refinements 

Refine cgroup v2 docs after latest memory.low changes.

Link: http://lkml.kernel.org/r/20180405185921.4942-4-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Roman Gushchin 2018-06-07 17:06:29 -07:00 committed by Linus Torvalds
parent 5f93ad6743
commit 7854207fe9
1 changed files with 13 additions and 15 deletions
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28
Documentation/admin-guide/cgroup-v2.rst
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@ -1005,10 +1005,17 @@ PAGE_SIZE multiple when read back.
A read-write single value file which exists on non-root A read-write single value file which exists on non-root
cgroups. The default is "0". cgroups. The default is "0".
Best-effort memory protection. If the memory usages of a Best-effort memory protection. If the memory usage of a
cgroup and all its ancestors are below their low boundaries, cgroup is within its effective low boundary, the cgroup's
the cgroup's memory won't be reclaimed unless memory can be memory won't be reclaimed unless memory can be reclaimed
reclaimed from unprotected cgroups. from unprotected cgroups.
Effective low boundary is limited by memory.low values of
all ancestor cgroups. If there is memory.low overcommitment
(child cgroup or cgroups are requiring more protected memory,
than parent will allow), then each child cgroup will get
the part of parent's protection proportional to the its
actual memory usage below memory.low.
Putting more memory than generally available under this Putting more memory than generally available under this
protection is discouraged. protection is discouraged.
@ -1950,17 +1957,8 @@ system performance due to overreclaim, to the point where the feature
becomes self-defeating. becomes self-defeating.
The memory.low boundary on the other hand is a top-down allocated The memory.low boundary on the other hand is a top-down allocated
reserve. A cgroup enjoys reclaim protection when it and all its reserve. A cgroup enjoys reclaim protection when it's within its low,
ancestors are below their low boundaries, which makes delegation of which makes delegation of subtrees possible.
subtrees possible. Secondly, new cgroups have no reserve per default
and in the common case most cgroups are eligible for the preferred
reclaim pass. This allows the new low boundary to be efficiently
implemented with just a minor addition to the generic reclaim code,
without the need for out-of-band data structures and reclaim passes.
Because the generic reclaim code considers all cgroups except for the
ones running low in the preferred first reclaim pass, overreclaim of
individual groups is eliminated as well, resulting in much better
overall workload performance.
The original high boundary, the hard limit, is defined as a strict The original high boundary, the hard limit, is defined as a strict
limit that can not budge, even if the OOM killer has to be called. limit that can not budge, even if the OOM killer has to be called.