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

Mike Galbraith
2012-09-19T12:35:30Z

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.

-Mike