linux-kernel - Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr

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Message-ID: <YVwqphxxNSDL828Y@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Oct 2021 12:36:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Mike Galbraith <efault@....de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running

On Thu, Sep 23, 2021 at 08:24:03AM -0400, Phil Auld wrote:

> It's capped at 8 cpus, which is pretty easy to reach these days, so the
> values don't get too large.  That scaling is almost a no-op these days.

  https://lkml.kernel.org/r/YVwdrh5pg0zSv2/b@hirez.programming.kicks-ass.net

Ooh, hey, we already fixed that :-)

So the reasoning there is that if the values get too big, interactiviy
get *really* bad, but if you go from say 1 to 4 CPUs, interactivity can
improve due to being able to run on other CPUs.

At 8 CPUs we end up at 6ms*4=24ms, which is already pretty terrible.