linux-kernel - Re: [PATCH] sched/numa: use runnable

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Date:   Fri, 28 Aug 2020 08:47:51 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/numa: use runnable_avg to classify node

On Thu, 27 Aug 2020 at 20:22, Mel Gorman <mgorman@...e.de> wrote:
>
> On Thu, Aug 27, 2020 at 05:43:11PM +0200, Vincent Guittot wrote:
> > > The testing was a mixed bag of wins and losses but wins more than it
> > > loses. Biggest loss was a 9.04% regression on nas-SP using openmp for
> > > parallelisation on Zen1. Biggest win was around 8% gain running
> > > specjbb2005 on Zen2 (with some major gains of up to 55% for some thread
> > > counts). Most workloads were stable across multiple Intel and AMD
> > > machines.
> > >
> > > There were some oddities in changes in NUMA scanning rate but that is
> > > likely a side-effect because the locality over time for the same loads
> > > did not look obviously worse. There was no negative result I could point
> > > at that was not offset by a positive result elsewhere. Given it's not
> > > a univeral win or loss, matching numa and lb balancing as closely as
> > > possible is best so
> > >
> > > Reviewed-by: Mel Gorman <mgorman@...e.de>
> >
> > Thanks.
> >
> > I will try to reproduce the nas-SP test on my setup to see what is going one
> >
>
> You can try but you might be chasing ghosts. Please note that this nas-SP
> observation was only on zen1 and only for C-class and OMP. The other
> machines tested for the same class and OMP were fine (including zen2). Even
> D-class on the same machine with OMP was fine as was MPI in both cases. The
> bad result indicated that NUMA scanning and faulting was higher but that
> is more likely to be a problem with NUMA balancing than your patch.
>
> In the five iterations, two iterations showed a large spike in scan rate
> towards the end of an iteration but not the other three. The scan rate
> was also not consistently high so there is a degree of luck involved with
> SP specifically and there is not a consistently penalty as a result of
> your patch.
>
> The only thing to be aware of is that this patch might show up in
> bisections once it's merged for both performance gains and losses.

Thanks for the detailed explanation. I will save my time and continue
on the fairness problem in this case.

Vincent

>
> --
> Mel Gorman
> SUSE Labs