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

[Mike Galbraith]

On Wed, 2012-09-19 at 16:54 +0200, Ingo Molnar wrote: 
> * Mike Galbraith <efault@xxxxxx> 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?

schedctl -B started postgress SCHED_BATCH, so pgbench won't be preempted
since it's the only SCHED_NORMAL task left in the lot.  All others are
postmaster, and SCHED_BATCH.

-Mike

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