Re: EEVDF and NUMA balancing

[Julia Lawall <julia.lawall@inria.fr>]

On Wed, 4 Oct 2023, Ingo Molnar wrote:

>
> * Julia Lawall <julia.lawall@inria.fr> wrote:
>
> >
> >
> > On Wed, 4 Oct 2023, Peter Zijlstra wrote:
> >
> > > On Wed, Oct 04, 2023 at 02:01:26PM +0200, Julia Lawall wrote:
> > > >
> > > >
> > > > On Tue, 3 Oct 2023, Peter Zijlstra wrote:
> > > >
> > > > > On Tue, Oct 03, 2023 at 10:25:08PM +0200, Julia Lawall wrote:
> > > > > > Is it expected that the commit e8f331bcc270 should have an impact on the
> > > > > > frequency of NUMA balancing?
> > > > >
> > > > > Definitely not expected. The only effect of that commit was supposed to
> > > > > be the runqueue order of tasks. I'll go stare at it in the morning --
> > > > > definitely too late for critical thinking atm.
> > > >
> > > > Maybe it's just randomly making a bad situation worse rather than directly
> > > > introduing a problem.  There is a high standard deviatind in the
> > > > performance.  Here are some results with hyperfine.  The general trends
> > > > are reproducible.
> > >
> > > OK,. I'm still busy trying to bring a 4 socket machine up-to-date...
> > > gawd I hate the boot times on those machines :/
> > >
> > > But yeah, I was thinking similar things, I really can't spot an obvious
> > > fail in that commit.
> > >
> > > I'll go have a poke once the darn machine is willing to submit :-)
> >
> > I tried a two-socket machine, but in 50 runs the problem doesn't show up.
> >
> > The commit e8f331bcc270 starts with
> >
> > -       if (sched_feat(PLACE_LAG) && cfs_rq->nr_running > 1) {
> > +       if (sched_feat(PLACE_LAG) && cfs_rq->nr_running) {
> >
> > This seemed like a big change - cfs_rq->nr_running > 1 should be rarely
> > true in ua, while cfs_rq->nr_running should always be true.  Adding back
> > the > 1 and simply replacing the test by 0 both had no effect, though.
>
> BTW., in terms of statistical reliability, one of the biggest ...
> stochastic elements of scheduler balancing is wakeup-preemption - which
> you can turn off via:
>
>    echo NO_WAKEUP_PREEMPTION > /debug/sched/features
>
> or:
>
>    echo NO_WAKEUP_PREEMPTION > /sys/kernel/debug/sched/features
>
> If you can measure a performance regression with WAKEUP_PREEMPTION turned
> off in *both* kernels, there's likely a material change (regression) in the
> quality of NUMA load-balancing.

This is the one that is the case.  In 76cae9dbe185, which is the parent of
e8f331bcc270, there are some runtimes that are a bit slower than with
WAKEUP_PREEMPTION enabled, but e8f331bcc270 now has a lot more larger
numbers.

julia

::::::::::::::
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::::::::::::::
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::::::::::::::
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>
> If it goes away or changes dramatically with WAKEUP_PREEMPTION off, then
> I'd pin this effect to EEVDF causing timing changes that are subtly
> shifting NUMA & SMP balancing decisions past some critical threshold that
> is detrimental to this particular workload.
>
> ( Obviously both are regressions we care about - but doing this test would
>   help categorize the nature of the regression. )
>
> Thanks,
>
> 	Ingo
>