Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected

Peter Zijlstra
2012-09-24T16:55:08Z

On Mon, 2012-09-24 at 09:30 -0700, Linus Torvalds wrote:
> On Mon, Sep 24, 2012 at 9:12 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >
> > So we're looking for an idle cpu around @target. We prefer a cpu of an
> > idle core, since SMT-siblings share L[12] cache. The way we do this is
> > by iterating the topology tree downwards starting at the LLC (L3) cache
> > level. Its groups are either the SMT-siblings or singleton groups.
>
> So if it'sally guaranteed to be SMT-siblings or singleton groups, then
> the whole "for_each_cpu()" is a total disaster. That's a truly
> expensive way to look up adjacent CPU's. Is there no saner way to look
> up that thing? Like a simple circular list of SMT siblings (I realize
> that on x86 that list is either one or two, but other SMT
> implementations are groups of four or more).

SMT siblings aren't actually adjacent in the cpu number space (on x86 at
least).

So the alternative you suggest is pointer chasing a list, is that really
much better than scanning a mostly empty bitmap?

I've no idea how bad these bitmap scanning instructions are on modern
chips. But let me try and come up with the list thing, I think we've
actually got that someplace as well.

> So I suspect your patch largely makes things faster (avoid those
> insane cpumask operations), but the for_each_cpu() one is still an
> absolutely horrible way to find a couple of basically statically known
> (modulo hotplug, which is disabled here anyway) CPU's. So even if the
> algorithm makes sense at some higher level, it doesn't really seem to
> make sense from an implementation standpoint.

Agreed.

> Also, do we really want to spread things out that aggressively?
> How/why do we know that we don't want to share L2 caches, for example?
> It sounds like a bad idea from a power standpoint, and possibly
> performance too.

IIRC this current stuff is the result of Mike and Suresh running a few
benchmarks.. Mike, Suresh, either one of you remember this? Otherwise
I'll have to go trawl the archives.