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: <CA+55aFyxjdir-LZneBTk1JQV0cVSRFzEK9Jpxje2SJqA6aP3Yg@mail.gmail.com>
Date:	Thu, 27 Sep 2012 10:44:26 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Mike Galbraith <efault@....de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mgorman@...e.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Thu, Sep 27, 2012 at 12:10 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> Just in case someone prefers patches to user-space approaches (I
> certainly do!), here's one that turns off wakeup driven
> preemption by default.

Ok, so apparently this fixes performance in a big way, and might allow
us to simplify select_idle_sibling(), which is clearly way too random.

That is, if we could make it automatic, some way. Not the "let the
user tune it" - that's just fundamentally broken.

What is the common pattern for the wakeups for psql?

Can we detect this somehow? Are they sync? It looks wrong to preempt
for sync wakeups, for example, but we seem to do that.

Or could we just improve the heuristics. What happens if the
scheduling granularity is increased, for example? It's set to 1ms
right now, with a logarithmic scaling by number of cpus.

        Linus
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