linux-kernel - Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

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Date:   Wed, 18 Jul 2018 17:37:43 +0200
From:   Bruce Merry <bmerry@....ac.za>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

On 18 July 2018 at 17:26, Shakeel Butt <shakeelb@...gle.com> wrote:
> On Wed, Jul 18, 2018 at 7:29 AM Bruce Merry <bmerry@....ac.za> wrote:
> It seems like you are using cgroup-v1. How many nodes are there in
> your memcg tree and also how many cpus does the system have?

>From my original email: "there are 106 memory.stat files in
/sys/fs/cgroup/memory." - is that what you mean by the number of
nodes?

The affected systems all have 8 CPU cores (hyperthreading is disabled).

> Please note that memcg_stat_show or reading memory.stat in cgroup-v1
> is not optimized as cgroup-v2. The function memcg_stat_show() in 4.13
> does ~17 tree walks and then for ~12 of those tree walks, it goes
> through all cpus for each node in the memcg tree. In 4.16,
> a983b5ebee57 ("mm: memcontrol: fix excessive complexity in memory.stat
> reporting") optimizes aways the cpu traversal at the expense of some
> accuracy. Next optimization would be to do just one memcg tree
> traversal similar to cgroup-v2's memory_stat_show().

On most machines it is still fast (1-2ms), and there is no difference
in the number of CPUs and only very small differences in the number of
live memory cgroups, so presumably something else is going on.

> The memcg tree does include all zombie memcgs and these zombies does
> contribute to the memcg_stat_show cost.

That sounds promising. Is there any way to tell how many zombies there
are, and is there any way to deliberately create zombies? If I can
produce zombies that might give me a reliable way to reproduce the
problem, which could then sensibly be tested against newer kernel
versions.

Thanks
Bruce
-- 
Bruce Merry
Senior Science Processing Developer
SKA South Africa