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Date: Wed, 19 Sep 2012 16:54:54 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, Andi Kleen <andi@firstfloor.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Nikolay Ulyanitsky <lystor@gmail.com>,
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        Andreas Herrmann <andreas.herrmann3@amd.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
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* 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