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

Ingo Molnar
2012-09-19T14:55:07Z

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

> On Sun, 2012-09-16 at 06:35 +0200, Mike Galbraith 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. IIRC pgbench _was_ at least 1:N,
> > ie one process driving the whole load. Waker of many
> > (singularly bad idea as a way to generate load) being
> > preempted by it's wakees stalls the whole load, so expensive
> > spreading of wakees to the four winds ala WAKE_BALANCE
> > becomes attractive, that pain being markedly less intense
> > than having multiple cores go idle while creator or work
> > waits for one.
>
> Enabling SMT on little E5620 box says that's the deal.
> pgbench as run is 1:N, and all you have to do is disable
> select_idle_sibling() entirely to see that for _this_ (~odd)
> load, max spread and lower wakeup latency for the mother of
> all work itself is a good thing.
>
> pgbench -i pgbench && pgbench -c $N -T 10 pgbench
>
> N= 1 2 4 8 16 32 64
> 1336 2482 3752 3485 3327 2928 2290 virgin 3.6.0-rc6
> 1408 2457 3363 3070 2938 2368 1757 +revert reverted
> 1310 2492 2487 2729 2186 975 874 +revert + select_idle_sibling() disabled
> 1407 2505 3422 3137 3093 2828 2250 +revert + schedctl -B /etc/init.d/postgresql restart
> 1321 2403 2515 2759 2420 2301 1894 +revert + schedctl -B /etc/init.d/postgresql restart + select_idle_sibling() disabled
>
> Hohum, damned if ya do, damned if ya don't. Damn.

As a test, could you mark that 'big PostgreSQL central work
queue process' with some high priority (renice -20?), to make
sure it's never preempted by wakees? Does that recover
performance as well?

Thanks,

Ingo