linux-kernel - Re: [PATCH] memcg: reduce memcg tree traversals for stats collection

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Date:   Wed, 25 Jul 2018 07:27:50 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     bmerry@....ac.za
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] memcg: reduce memcg tree traversals for stats collection

On Wed, Jul 25, 2018 at 4:26 AM Bruce Merry <bmerry@....ac.za> wrote:
>
> On 25 July 2018 at 00:46, Shakeel Butt <shakeelb@...gle.com> wrote:
> > I ran a simple benchmark which reads the root_mem_cgroup's stat file
> > 1000 times in the presense of 2500 memcgs on cgroup-v1. The results are:
> >
> > Without the patch:
> > $ time ./read-root-stat-1000-times
> >
> > real    0m1.663s
> > user    0m0.000s
> > sys     0m1.660s
> >
> > With the patch:
> > $ time ./read-root-stat-1000-times
> >
> > real    0m0.468s
> > user    0m0.000s
> > sys     0m0.467s
>
> Thanks for cc'ing me. I've tried this patch using my test case and the
> results are interesting. With the patch applied, running my script
> only generates about 8000 new cgroups, compared to 40,000 before -
> presumably because the optimisation has altered the timing.
>
> On the other hand, if I run the script 5 times to generate 40000
> zombie cgroups, the time to get stats for the root cgroup (cgroup-v1)
> is almost unchanged at around 18ms (was 20ms, but there were slightly
> more cgroups as well), compared to the almost 4x speedup you're seeing
> in your test.
>

Hi Bruce, I think your script is trying to create zombies, so, the
experiments after that script will be non-deterministic. Why not just
create 40k cgroups ,no need for zombies, and the see how much this
patch affects reading stats.

Shakeel