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

Mike Galbraith
2012-09-17T10:47:33Z

On Mon, 2012-09-17 at 12:07 +0200, Ingo Molnar wrote:
> * Mike Galbraith <efault@gmx.de> wrote:
>

> > 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?

It's been a while since I fiddled with oltp (lost my fast mysql db,
every attempt to re-create produced a complete slug), but postgress was
always the throughput winner at that here.

> > 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.

I don't know what a v2 would look like, but I can keep thinking about
this irritating little <naughty words elided>. Peter's a lot hairier
chested, not to mention having a sense of _taste_ :) so it might be
better to just consider my patch a diagnostic, and let him fix it up in
a (likely lots) less tummy distressing manner.

-Mike