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Date:   Tue, 3 Oct 2023 10:09:48 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] mm: memcg: fix tracking of pending stats updates
 values
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On Tue 03-10-23 01:03:53, Yosry Ahmed wrote:
> On Tue, Oct 3, 2023 at 12:57 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 25-09-23 10:11:05, Yosry Ahmed wrote:
> > > On Mon, Sep 25, 2023 at 6:50 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Fri 22-09-23 17:57:38, Yosry Ahmed wrote:
> > > > > While working on adjacent code [1], I realized that the values passed
> > > > > into memcg_rstat_updated() to keep track of the magnitude of pending
> > > > > updates is consistent. It is mostly in pages, but sometimes it can be in
> > > > > bytes or KBs. Fix that.
> > > >
> > > > What kind of practical difference does this change make? Is it worth
> > > > additional code?
> > >
> > > As explained in patch 2's commit message, the value passed into
> > > memcg_rstat_updated() is used for the "flush only if not worth it"
> > > heuristic. As we have discussed in different threads in the past few
> > > weeks, unnecessary flushes can cause increased global lock contention
> > > and/or latency.
> > >
> > > Byte-sized paths (percpu, slab, zswap, ..) feed bytes into the
> > > heuristic, but those are interpreted as pages, which means we will
> > > flush earlier than we should. This was noticed by code inspection. How
> > > much does this matter in practice? I would say it depends on the
> > > workload: how many percpu/slab allocations are being made vs. how many
> > > flushes are requested.
> > >
> > > On a system with 100 cpus, 25M of stat updates are needed for a flush
> > > usually, but ~6K of slab/percpu updates will also (mistakenly) cause a
> > > flush.
> >
> > This surely depends on workload and that is understandable. But it would
> > be really nice to provide some numbers for typical workloads which
> > exercise slab heavily.
> 
> If you have a workload in mind I can run it and see how many flushes
> we get with/without this patch. The first thing that pops into my head
> is creating a bunch of empty files but I don't know if that's the best
> thing to get numbers from.

Let me remind you that you are proposing a performance optimization and
such a change requires some numbers to actually show it is benefitial.
There are cases where the resulting code is clearly an improvement and
the performance benefit is just a nice side effect. I do not consider
this to be the case. The whole thing is quite convoluted even without
a better precision you are proposing. And let me be clear, I am not
opposing your patch but I would rather see it based on more than just
hand waving.
-- 
Michal Hocko
SUSE Labs