Commit 5647e53f authored by Dan Schatzberg's avatar Dan Schatzberg Committed by Tejun Heo
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cgroup: Documentation: Clarify usage of memory limits 



The existing documentation refers to memory.high as the "main mechanism
to control memory usage." This seems incorrect to me - memory.high can
result in reclaim pressure which simply leads to stalls unless some
external component observes and actions on it (e.g. systemd-oomd can be
used for this purpose). While this is feasible, users are unaware of
this interaction and are led to believe that memory.high alone is an
effective mechanism for limiting memory.

The documentation should recommend the use of memory.max as the
effective way to enforce memory limits - it triggers reclaim and results
in OOM kills by itself.

Signed-off-by: default avatarDan Schatzberg <schatzberg.dan@gmail.com>
Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarChris Down <chris@chrisdown.name>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 2bd11033

Documentation/admin-guide/cgroup-v2.rst

+10 −12
Original line number Diff line number Diff line
@@ -1213,23 +1213,25 @@ PAGE_SIZE multiple when read back. 
	A read-write single value file which exists on non-root

	cgroups.  The default is "max".



	Memory usage throttle limit.  This is the main mechanism to

	control memory usage of a cgroup.  If a cgroup's usage goes

	Memory usage throttle limit.  If a cgroup's usage goes

	over the high boundary, the processes of the cgroup are

	throttled and put under heavy reclaim pressure.



	Going over the high limit never invokes the OOM killer and

	under extreme conditions the limit may be breached.

	under extreme conditions the limit may be breached. The high

	limit should be used in scenarios where an external process

	monitors the limited cgroup to alleviate heavy reclaim

	pressure.



  memory.max

	A read-write single value file which exists on non-root

	cgroups.  The default is "max".



	Memory usage hard limit.  This is the final protection

	mechanism.  If a cgroup's memory usage reaches this limit and

	can't be reduced, the OOM killer is invoked in the cgroup.

	Under certain circumstances, the usage may go over the limit

	temporarily.

	Memory usage hard limit.  This is the main mechanism to limit

	memory usage of a cgroup.  If a cgroup's memory usage reaches

	this limit and can't be reduced, the OOM killer is invoked in

	the cgroup. Under certain circumstances, the usage may go

	over the limit temporarily.



	In default configuration regular 0-order allocations always

	succeed unless OOM killer chooses current task as a victim.
@@ -1238,10 +1240,6 @@ PAGE_SIZE multiple when read back. 
	Caller could retry them differently, return into userspace

	as -ENOMEM or silently ignore in cases like disk readahead.



	This is the ultimate protection mechanism.  As long as the

	high limit is used and monitored properly, this limit's

	utility is limited to providing the final safety net.



  memory.reclaim

	A write-only nested-keyed file which exists for all cgroups.