linux-kernel - Re: [PATCH 2/2] sched/psi: iterate through cgroups directly

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Message-ID: <20230215182547.GA17571@blackbody.suse.cz>
Date:   Wed, 15 Feb 2023 19:25:48 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Kairui Song <ryncsn@...il.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Chengming Zhou <zhouchengming@...edance.com>,
        Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/psi: iterate through cgroups directly

On Thu, Feb 16, 2023 at 01:49:22AM +0800, Kairui Song <ryncsn@...il.com> wrote:
> With rootcg:
> 55718.9 op/s (unpatched) compared to 55862.2 (patched)
> With 5 levels:
> 49975.5 op/s (unpatched) compared to 50778.5 op/s (patched)
> 
> Previous tests are a bit biased since I only run the test for 100 * 3
> times, or maybe it is sensitive to some random kernel structure
> changes.
> 
> But I ignored one important thing in my previous test, that the
> performance actually drops heavily with deeper levers of cgroups:
> With 8 levels:
> 48902.4 op/s (unpatched) compared to 47476.6 op/s (patched)
> With 50 levels of cgroup:
> 25605.7375 op/s (unpatched) compared to 20417.275 op/s (patched)

IIUC, one could also interpret this as the parent caching within
psi_group is effective especially with deep hierarchies.

I'd say practical hierarchies are below 10 levels deep. But yeah, the
averaged results aren't so impressive.

Thanks for sharing your insights,
Michal

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