Re: [tip: sched/core] sched/fair: Multi-LLC select_idle_sibling()

Peter Zijlstra
2023-06-01T12:00:32Z

On Thu, Jun 01, 2023 at 01:56:43PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 01, 2023 at 01:13:26PM +0200, Peter Zijlstra wrote:
> >
> > This DeathStarBench thing seems to suggest that scanning up to 4 CCDs
> > isn't too much of a bother; so perhaps something like so?
> >
> > (on top of tip/sched/core from just a few hours ago, as I had to 'fix'
> > this patch and force pushed the thing)
> >
> > And yeah, random hacks and heuristics here :/ Does there happen to be
> > additional topology that could aid us here? Does the CCD fabric itself
> > have a distance metric we can use?
>
> https://www.anandtech.com/show/16529/amd-epyc-milan-review/4
>
> Specifically:
>
> https://images.anandtech.com/doci/16529/Bounce-7763.png
>
> That seems to suggest there are some very minor distance effects in the
> CCD fabric. I didn't read the article too closely, but you'll note that
> the first 4 CCDs have inter-CCD latency < 100 while the rest has > 100.
>
> Could you also test on a Zen2 Epyc, does that require nr=8 instead of 4?
> Should we perhaps write it like: 32 / llc_size ?
>
> The Zen2 picture:
>
> https://images.anandtech.com/doci/16315/Bounce-7742.png
>
> Shows a more pronounced CCD fabric topology, you can really see the 2
> CCX inside the CCD but also there's two ligher green squares around the
> CCDs themselves.

I can't seem to find pretty pictures for Zen4 Epyc; what does that want?
That's even bigger at 96/8=12 LLCs afaict.