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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 12 Oct 2023 14:05:30 -0700
Message-ID: <CAJD7tkbSwNOZu1r8VfUAD5v-g_NK3oASfO51FJDX4pdMYh9mjw@mail.gmail.com>
Subject: Re: [PATCH v2 3/5] mm: memcg: make stats flushing threshold per-memcg
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Ivan Babrou <ivan@cloudflare.com>, Tejun Heo <tj@kernel.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Waiman Long <longman@redhat.com>, kernel-team@cloudflare.com,
        Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>,
        linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
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[..]
> > > >
> > > > Using next-20231009 and a similar 44 core machine with hyperthreading
> > > > disabled, I ran 22 instances of netperf in parallel and got the
> > > > following numbers from averaging 20 runs:
> > > >
> > > > Base: 33076.5 mbps
> > > > Patched: 31410.1 mbps
> > > >
> > > > That's about 5% diff. I guess the number of iterations helps reduce
> > > > the noise? I am not sure.
> > > >
> > > > Please also keep in mind that in this case all netperf instances are
> > > > in the same cgroup and at a 4-level depth. I imagine in a practical
> > > > setup processes would be a little more spread out, which means less
> > > > common ancestors, so less contended atomic operations.
> > >
> > >
> > > (Resending the reply as I messed up the last one, was not in plain text)
> > >
> > > I was curious, so I ran the same testing in a cgroup 2 levels deep
> > > (i.e /sys/fs/cgroup/a/b), which is a much more common setup in my
> > > experience. Here are the numbers:
> > >
> > > Base: 40198.0 mbps
> > > Patched: 38629.7 mbps
> > >
> > > The regression is reduced to ~3.9%.
> > >
> > > What's more interesting is that going from a level 2 cgroup to a level
> > > 4 cgroup is already a big hit with or without this patch:
> > >
> > > Base: 40198.0 -> 33076.5 mbps (~17.7% regression)
> > > Patched: 38629.7 -> 31410.1 (~18.7% regression)
> > >
> > > So going from level 2 to 4 is already a significant regression for
> > > other reasons (e.g. hierarchical charging). This patch only makes it
> > > marginally worse. This puts the numbers more into perspective imo than
> > > comparing values at level 4. What do you think?
> >
> > This is weird as we are running the experiments on the same machine. I
> > will rerun with 2 levels as well. Also can you rerun the page fault
> > benchmark as well which was showing 9% regression in your original
> > commit message?
>
> Thanks. I will re-run the page_fault tests, but keep in mind that the
> page fault benchmarks in will-it-scale are highly variable. We run
> them between kernel versions internally, and I think we ignore any
> changes below 10% as the benchmark is naturally noisy.
>
> I have a couple of runs for page_fault3_scalability showing a 2-3%
> improvement with this patch :)

I ran the page_fault tests for 10 runs on a machine with 256 cpus in a
level 2 cgroup, here are the results (the results in the original
commit message are for 384 cpus in a level 4 cgroup):

               LABEL            |     MEAN    |   MEDIAN    |   STDDEV   |
------------------------------+-------------+-------------+-------------
  page_fault1_per_process_ops |             |             |            |
  (A) base                    | 270249.164  | 265437.000  | 13451.836  |
  (B) patched                 | 261368.709  | 255725.000  | 13394.767  |
                              | -3.29%      | -3.66%      |            |
  page_fault1_per_thread_ops  |             |             |            |
  (A) base                    | 242111.345  | 239737.000  | 10026.031  |
  (B) patched                 | 237057.109  | 235305.000  | 9769.687   |
                              | -2.09%      | -1.85%      |            |
  page_fault1_scalability     |             |             |
  (A) base                    | 0.034387    | 0.035168    | 0.0018283  |
  (B) patched                 | 0.033988    | 0.034573    | 0.0018056  |
                              | -1.16%      | -1.69%      |            |
  page_fault2_per_process_ops |             |             |
  (A) base                    | 203561.836  | 203301.000  | 2550.764   |
  (B) patched                 | 197195.945  | 197746.000  | 2264.263   |
                              | -3.13%      | -2.73%      |            |
  page_fault2_per_thread_ops  |             |             |
  (A) base                    | 171046.473  | 170776.000  | 1509.679   |
  (B) patched                 | 166626.327  | 166406.000  | 768.753    |
                              | -2.58%      | -2.56%      |            |
  page_fault2_scalability     |             |             |
  (A) base                    | 0.054026    | 0.053821    | 0.00062121 |
  (B) patched                 | 0.053329    | 0.05306     | 0.00048394 |
                              | -1.29%      | -1.41%      |            |
  page_fault3_per_process_ops |             |             |
  (A) base                    | 1295807.782 | 1297550.000 | 5907.585   |
  (B) patched                 | 1275579.873 | 1273359.000 | 8759.160   |
                              | -1.56%      | -1.86%      |            |
  page_fault3_per_thread_ops  |             |             |
  (A) base                    | 391234.164  | 390860.000  | 1760.720   |
  (B) patched                 | 377231.273  | 376369.000  | 1874.971   |
                              | -3.58%      | -3.71%      |            |
  page_fault3_scalability     |             |             |
  (A) base                    | 0.60369     | 0.60072     | 0.0083029  |
  (B) patched                 | 0.61733     | 0.61544     | 0.009855   |
                              | +2.26%      | +2.45%      |            |

The numbers are much better. I can modify the commit log to include
the testing in the replies instead of what's currently there if this
helps (22 netperf instances on 44 cpus and will-it-scale page_fault on
256 cpus -- all in a level 2 cgroup).