Re: [PATCH v5 2/2] sched/fair: Introduce SIS_SHORT to wake up short task on current CPU

[Abel Wu]

On 2/16/23 11:24 PM, Chen Yu wrote:
> > The following change greatly reduced the p99lat of Redis service
> > from 150ms to 0.9ms, at exactly the same throughput (QPS).
> >
> > @@ -5763,6 +5787,9 @@ wake_affine_weight(struct sched_domain *sd, struct
> > task_struct *p,
> > 	s64 this_eff_load, prev_eff_load;
> > 	unsigned long task_load;
> >
> > +	if (is_short_task(p))
> > +		return nr_cpumask_bits;
> > +
> So above change wants to wake up the short task on its previous
> CPU if I understand correctly.

Yes.

> > 	this_eff_load = cpu_load(cpu_rq(this_cpu));
> >
> > 	if (sync) {
> >
> > I know that 'short' tasks are not necessarily 'small' tasks, e.g.
> > sleeping duration is small or have large weights, but this works
> > really well for this case. This is partly because delivering data
> > is memory bandwidth intensive hence prefer cache hot cpus. And I
> > think this is also applicable to the general purposes: do NOT let
> > the short running tasks suffering from cache misses caused by
> > migration.
> I see. My original thought was to mitigate short task migration
> as much as possible. Either waking up the task on current CPU or previous
> CPU should both achieve the goal in theory. Could you please describe
> a little more about how Redis proxy server was tested? Was it tested
> locally or using multiple machines? I asked this because for network
> benchmarks, it might be better to wake the task close to the waker(maybe
> the NIC interrupt) due to hot network buffer. Anyway I will test
> your change slightly changed to see the impact, and also Redis. But it
> would be even better if you could provide some simple test steps I can
> try locally : )

Sorry for missing the info. The test was done in production environment,
and what I have done is only updating the kernel in several machines
which are highly loaded, that is over 85% cpu util observed by mpstat.
Please let me know if you want any specific info.

Best,
	Abel