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

Ingo Molnar
2012-09-17T10:08:09Z

* Mike Galbraith <efault@gmx.de> wrote:

> On Sun, 2012-09-16 at 12:57 -0700, Linus Torvalds wrote:
> > On Sat, Sep 15, 2012 at 9:35 PM, Mike Galbraith <efault@gmx.de> wrote:
> > >
> > > Oh, while I'm thinking about it, there's another scenario that could
> > > cause the select_idle_sibling() change to affect pgbench on largeish
> > > packages, but it boils down to preemption odds as well.
> >
> > So here's a possible suggestion..
> >
> > Let's assume that the scheduler code to find the next idle CPU on the
> > package is actually a good idea, and we shouldn't mess with the idea.
>
> We should definitely mess with the idea, as it causes some problems.
>
> > But at the same time, it's clearly an *expensive* idea,
> > which is why you introduced the "only test a single CPU
> > buddy" approach instead. But that didn't work, and you can
> > come up with multiple reasons why it wouldn't work. Plus,
> > quite fundamentally, it's rather understandable that "try to
> > find an idle CPU on the same package" really would be a good
> > idea, right?
>
> I would argue that it did work, it shut down the primary
> source of pain, which I do not believe to be the traversal
> cost, rather the bouncing.
>
> 4 socket 40 core + SMT Westmere box, single 30 sec tbench runs, higher is better:
>
> clients 1 2 4 8 16 32 64 128
> ..........................................................................
> pre 30 41 118 645 3769 6214 12233 14312
> post 299 603 1211 2418 4697 6847 11606 14557

That's a very tempting speedup for a simpler and more
fundamental workload than postgresql's somewhat weird
user-space spinlocks that burn CPU time in user-space
instead of blocking/waiting on a futex.

IIRC mysql does this properly and outperforms postgresql
on this benchmark, in an apples-to-apples configuration?

> 10x at 1 pair shouldn't be traversal, the whole box is
> otherwise idle. We'll do a lot more (ever more futile)
> traversal as load increases, but at the same time, our futile
> attempts fail more frequently, so we shoot ourselves in the
> foot less frequently.
>
> The down side is (appears to be) that I also shut down some
> ~odd case preemption salvation, salvation that only large
> packages will receive.
>
> The problem as I see it is that we're making light tasks _too_
> mobile, turning an optimization into a pessimization for light
> tasks. For longer running tasks this mobility within a large
> package isn't such a big deal, but for fast movers, it hurts a
> lot.

There's not enough time to resolve this for v3.6, so I agree
with the revert - would you be willing to post a v2 of your
original patch? I really think we want your tbench speedups,
quite a few real-world messaging applications use the tbench
patterns of scheduling.

Thanks,

Ingo