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: <CAFLxGvy_yu3L5HrsuF9A=4HpbB-5twjkaswUbb0PUhK9NjKtvA@mail.gmail.com>
Date:	Sat, 15 Sep 2012 19:08:59 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
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

On Sat, Sep 15, 2012 at 6:36 PM, Mike Galbraith <efault@....de> wrote:
>> Just using futex is unfortunately not the answer either.
>
> 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?

Why perform custom userspace spinlocks better than futex() based ones?
I thought we have futex() to get rid of the custom ones...
Makes futex() only sense when things like priority inheritance are needed?

-- 
Thanks,
//richard
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