linux-kernel - Re: [RFC PATCH 0/8] memcg: Enable fine-grained per process memory control

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Date:   Tue, 18 Aug 2020 12:04:44 +0200
From:   peterz@...radead.org
To:     Chris Down <chris@...isdown.name>
Cc:     Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/8] memcg: Enable fine-grained per process memory
 control

On Tue, Aug 18, 2020 at 10:27:37AM +0100, Chris Down wrote:
> peterz@...radead.org writes:
> > On Mon, Aug 17, 2020 at 10:08:23AM -0400, Waiman Long wrote:
> > > Memory controller can be used to control and limit the amount of
> > > physical memory used by a task. When a limit is set in "memory.high" in
> > > a v2 non-root memory cgroup, the memory controller will try to reclaim
> > > memory if the limit has been exceeded. Normally, that will be enough
> > > to keep the physical memory consumption of tasks in the memory cgroup
> > > to be around or below the "memory.high" limit.
> > > 
> > > Sometimes, memory reclaim may not be able to recover memory in a rate
> > > that can catch up to the physical memory allocation rate. In this case,
> > > the physical memory consumption will keep on increasing.
> > 
> > Then slow down the allocator? That's what we do for dirty pages too, we
> > slow down the dirtier when we run against the limits.
> 
> We already do that since v5.4. I'm wondering whether Waiman's customer is
> just running with a too-old kernel without 0e4b01df865 ("mm, memcg: throttle
> allocators when failing reclaim over memory.high") backported.

That commit is fundamentally broken, it doesn't guarantee anything.

Please go read how the dirty throttling works (unless people wrecked
that since..).