linux-kernel - Re: [PATCH] Documentation: Clarify usage of memory limits

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Message-ID: <ZHuxvjP4QlsT1saH@chrisdown.name>
Date:   Sat, 3 Jun 2023 22:33:50 +0100
From:   Chris Down <chris@...isdown.name>
To:     Dan Schatzberg <schatzberg.dan@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation: Clarify usage of memory limits

Dan Schatzberg writes:
>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: Dan Schatzberg <schatzberg.dan@...il.com>

Oof, the documentation is very out of date indeed -- no wonder people were 
confused by other advice to only use memory.high with something external 
monitoring the cgroup.

Thanks!

Acked-by: Chris Down <chris@...isdown.name>

>---
> Documentation/admin-guide/cgroup-v2.rst | 22 ++++++++++------------
> 1 file changed, 10 insertions(+), 12 deletions(-)
>
>diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>index f67c0829350b..e592a9364473 100644
>--- a/Documentation/admin-guide/cgroup-v2.rst
>+++ b/Documentation/admin-guide/cgroup-v2.rst
>@@ -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.
>
>-- 
>2.34.1
>