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: <20120915223212.4174a314@pyramind.ukuu.org.uk>
Date:	Sat, 15 Sep 2012 22:32:12 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mike Galbraith <efault@....de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

> Yes, postgress performs loads better with it's spinlocks, but due to
> that, it necessarily _hates_ preemption.  How the is the scheduler
> supposed to know that any specific userland task _really_ shouldn't be
> preempted at any specific time, else bad things follow?

You provide a shared page for a process group so it can write hints to
which is kernel mapped so the scheduler can peek..

Alan
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