Messages in this thread | | | Date | Wed, 4 Oct 2023 20:15:10 +0200 | From | Ingo Molnar <> | Subject | Re: EEVDF and NUMA balancing |
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* 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.
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
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