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

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Message-ID: <20120915194453.GA30632@liondog.tnic>
Date:	Sat, 15 Sep 2012 21:44:54 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Mike Galbraith <efault@....de>
Cc:	Nikolay Ulyanitsky <lystor@...il.com>,
	linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Sat, Sep 15, 2012 at 06:13:45PM +0200, Mike Galbraith wrote:
> > Just for my n00b scheduler understanding: this way you're practically
> > extending the timeslice of the task so that it gets done without being
> > preempted and the lock-holding period of the preempted task gets smaller
> > and thus you get more completed transactions in postgres during the
> > benchmark run?
> 
> Not really, preemption will happen, but when the preempting task goes to
> sleep (or uses it's fair share), instead of selecting the leftmost task
> (lowest vruntime), the preempted task gets the CPU back if we can do
> that without violating fairness.  If the preempted task happens to be a
> userland spinlock holder, it then release the lock sooner, others don't
> spin as long, do more work, less playing space heater while lock holder
> waits for spinners to eat enough CPU to become less deserving that it.

Ok, I definitely grok this. Thanks for explaining.

-- 
Regards/Gruss,
    Boris.
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