Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to3.6-rc5 on AMD chipsets - bisected

[Mike Galbraith]

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 


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