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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Date:	Thu, 27 Sep 2012 11:19:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....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 11:05 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, Sep 27, 2012 at 10:44:26AM -0700, Linus Torvalds wrote:
>> 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.
>
> /proc/sys/kernel/sched_wakeup_granularity_ns=10000000 (10ms)
> ------------------------------------------------------
> tps = 4994.730809 (including connections establishing)
> tps = 5000.260764 (excluding connections establishing)
>
> A bit better over the default NO_WAKEUP_PREEMPTION setting.

Ok, so this gives us something possible to actually play with.

For example, maybe SCHED_TUNABLESCALING_LINEAR is more appropriate
than SCHED_TUNABLESCALING_LOG. At least for WAKEUP_PREEMPTION. Hmm?

(Btw, "linear" right now looks like 1:1. That's linear, but it's a
very aggressive linearity. Something like "factor = (cpus+1)/2" would
also be linear, but by a less extreme factor.

              Linus
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