linux-kernel - Re: [PATCH 4/4] sched/fair: reduce busy load balance interval

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Message-ID: <CAPJCdBmy9+0aQuhvxP3vjU+o6XCKpfLDpbesDLsgRrF_xsKCEg@mail.gmail.com>
Date:   Wed, 16 Sep 2020 09:14:13 +0800
From:   Jiang Biao <benbjiang@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH 4/4] sched/fair: reduce busy load balance interval

Hi,

On Tue, 15 Sep 2020 at 20:43, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Tue, 15 Sep 2020 at 13:36, Jiang Biao <benbjiang@...il.com> wrote:
> >
> > Hi, Vincent
> >
> > On Tue, 15 Sep 2020 at 17:28, Vincent Guittot
> > <vincent.guittot@...aro.org> wrote:
> > >
> > > On Tue, 15 Sep 2020 at 11:11, Jiang Biao <benbjiang@...il.com> wrote:
> > > >
> > > > Hi, Vincent
> > > >
> > > > On Mon, 14 Sep 2020 at 18:07, Vincent Guittot
> > > > <vincent.guittot@...aro.org> wrote:
> > > > >
> > > > > The busy_factor, which increases load balance interval when a cpu is busy,
> > > > > is set to 32 by default. This value generates some huge LB interval on
> > > > > large system like the THX2 made of 2 node x 28 cores x 4 threads.
> > > > > For such system, the interval increases from 112ms to 3584ms at MC level.
> > > > > And from 228ms to 7168ms at NUMA level.
> > > > Agreed that the interval is too big for that case.
> > > > But would it be too small for an AMD environment(like ROME) with 8cpu
> > > > at MC level(CCX), if we reduce busy_factor?
> > >
> > > Are you sure that this is too small ? As mentioned in the commit
> > > message below, I tested it on small system (2x4 cores Arm64) and i
> > > have seen some improvements
> > Not so sure. :)
> > Small interval means more frequent balances and more cost consumed for
> > balancing, especially for pinned vm cases.
>
> If you are running only pinned threads, the interval can increase
> above 512ms which means 8sec after applying the busy factor
Yep. :)

>
> > For our case, we have AMD ROME servers made of 2node x 48cores x
> > 2thread, and 8c at MC level(within a CCX). The 256ms interval seems a
> > little too big for us, compared to Intel Cascadlake CPU with 48c at MC
>
> so IIUC your topology is :
> 2 nodes at NUMA
> 6 CCX at DIE level
> 8 cores per CCX at MC
> 2 threads per core at SMT
Yes.

>
> > level, whose balance interval is 1536ms. 128ms seems a little more
> > waste. :)
>
> the 256ms/128ms interval only looks at 8 cores whereas the 1536
> intervall looks for the whole 48 cores
Yes. The real problem for us is the cpu number difference between MC
and DIE level is too big(8 VS. 96), 3072ms for DIE level is too big(reduce
busy_factor is good enough), while 128ms for MC level seems a little waste(
if reduce busy_factor)
And no objection for this patch. It still looks ok for us.

Thx.
Regards,
Jiang