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

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Message-ID: <YVxdPGj92+FcVrgt@lorien.usersys.redhat.com>
Date:   Tue, 5 Oct 2021 10:12:12 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Tue, Oct 05, 2021 at 12:36:22PM +0200 Peter Zijlstra wrote:
> 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 :-)
>

Thanks Peter.

I'm always a little behind upstream (nature of the job :)

That link leads to a message Id not found. But from what I can see the
code that takes the min of online cpus and 8 is still present. 


> 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.
> 

And actually you mention the same thing later on.  Most systems, even
desktops, have 8+ cpus these days so the scaling is mostly not doing
anything except multiplying by 4, right? So no-op was not the right
way to describe it maybe. But it's not getting bigger with larger
numbers of cpus beyond a pretty commonly reached limit.


Cheers,
Phil

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