linux-kernel - Re: [PATCH] memcg: Add a new sysctl parameter for automatically setting memory.high

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Date: Mon, 24 Jun 2024 08:21:22 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Waiman Long <longman@...hat.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Shakeel Butt <shakeel.butt@...ux.dev>, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	Alex Kalenyuk <akalenyu@...hat.com>, Peter Hunt <pehunt@...hat.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH] memcg: Add a new sysctl parameter for automatically
 setting memory.high

On Sun, Jun 23, 2024 at 04:52:00PM -0400, Waiman Long wrote:
> Correct some email addresses.
> 
> On 6/23/24 16:45, Waiman Long wrote:
> > With memory cgroup v1, there is only a single "memory.limit_in_bytes"
> > to be set to specify the maximum amount of memory that is allowed to
> > be used. So a lot of memory cgroup using tools and applications allow
> > users to specify a single memory limit. When they migrate to cgroup
> > v2, they use the given memory limit to set memory.max and disregard
> > memory.high for the time being.
> > 
> > Without properly setting memory.high, these user space applications
> > cannot make use of the memory cgroup v2 ability to further reduce the
> > chance of OOM kills by throttling and early memory reclaim.
> > 
> > This patch adds a new sysctl parameter "vm/memory_high_autoset_ratio"
> > to enable setting "memory.high" automatically whenever "memory.max" is
> > set as long as "memory.high" hasn't been explicitly set before. This
> > will allow a system administrator or a middleware layer to greatly
> > reduce the chance of memory cgroup OOM kills without worrying about
> > how to properly set memory.high.
> > 
> > The new sysctl parameter will allow a range of 0-100. The default value
> > of 0 will disable memory.high auto setting. For any non-zero value "n",
> > the actual ratio used will be "n/(n+1)". A user cannot set a fraction
> > less than 1/2.

Hi Waiman,

I'm not sure that setting memory.high is always a good idea (it comes
with a certain cost, e.g. can increase latency), but even if it is,
why systemd or similar userspace tools can't do this?

I wonder what's special about your case if you do see a lot of OOMs
which can be avoided by setting memory.high? Do you have a bursty workload?

Thanks!