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Date:   Thu, 12 Oct 2023 09:29:46 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.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>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <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
Subject: Re: [PATCH v2 3/5] mm: memcg: make stats flushing threshold per-memcg
Message-ID: <20231012132946.GA470544@cmpxchg.org>
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On Thu, Oct 12, 2023 at 01:04:03AM -0700, Yosry Ahmed wrote:
> On Wed, Oct 11, 2023 at 8:13 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Wed, Oct 11, 2023 at 5:46 AM Shakeel Butt <shakeelb@google.com> wrote:
> > >
> > > On Tue, Oct 10, 2023 at 6:48 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > >
> > > > On Tue, Oct 10, 2023 at 5:36 PM Shakeel Butt <shakeelb@google.com> wrote:
> > > > >
> > > > > On Tue, Oct 10, 2023 at 03:21:47PM -0700, Yosry Ahmed wrote:
> > > > > [...]
> > > > > >
> > > > > > I tried this on a machine with 72 cpus (also ixion), running both
> > > > > > netserver and netperf in /sys/fs/cgroup/a/b/c/d as follows:
> > > > > > # echo "+memory" > /sys/fs/cgroup/cgroup.subtree_control
> > > > > > # mkdir /sys/fs/cgroup/a
> > > > > > # echo "+memory" > /sys/fs/cgroup/a/cgroup.subtree_control
> > > > > > # mkdir /sys/fs/cgroup/a/b
> > > > > > # echo "+memory" > /sys/fs/cgroup/a/b/cgroup.subtree_control
> > > > > > # mkdir /sys/fs/cgroup/a/b/c
> > > > > > # echo "+memory" > /sys/fs/cgroup/a/b/c/cgroup.subtree_control
> > > > > > # mkdir /sys/fs/cgroup/a/b/c/d
> > > > > > # echo 0 > /sys/fs/cgroup/a/b/c/d/cgroup.procs
> > > > > > # ./netserver -6
> > > > > >
> > > > > > # echo 0 > /sys/fs/cgroup/a/b/c/d/cgroup.procs
> > > > > > # for i in $(seq 10); do ./netperf -6 -H ::1 -l 60 -t TCP_SENDFILE --
> > > > > > -m 10K; done
> > > > >
> > > > > You are missing '&' at the end. Use something like below:
> > > > >
> > > > > #!/bin/bash
> > > > > for i in {1..22}
> > > > > do
> > > > >    /data/tmp/netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K &
> > > > > done
> > > > > wait
> > > > >
> > > >
> > > > Oh sorry I missed the fact that you are running instances in parallel, my bad.
> > > >
> > > > So I ran 36 instances on a machine with 72 cpus. I did this 10 times
> > > > and got an average from all instances for all runs to reduce noise:
> > > >
> > > > #!/bin/bash
> > > >
> > > > ITER=10
> > > > NR_INSTANCES=36
> > > >
> > > > for i in $(seq $ITER); do
> > > >   echo "iteration $i"
> > > >   for j in $(seq $NR_INSTANCES); do
> > > >     echo "iteration $i" >> "out$j"
> > > >     ./netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K >> "out$j" &
> > > >   done
> > > >   wait
> > > > done
> > > >
> > > > cat out* | grep 540000 | awk '{sum += $5} END {print sum/NR}'
> > > >
> > > > Base: 22169 mbps
> > > > Patched: 21331.9 mbps
> > > >
> > > > The difference is ~3.7% in my runs. I am not sure what's different.
> > > > Perhaps it's the number of runs?
> > >
> > > My base kernel is next-20231009 and I am running experiments with
> > > hyperthreading disabled.
> >
> > 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?

I think it's reasonable.

Especially comparing to how many cachelines we used to touch on the
write side when all flushing happened there. This looks like a good
trade-off to me.