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Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Mike Galbraith <efault@gmx.de>, Mel Gorman <mgorman@suse.de>,
        Nikolay Ulyanitsky <lystor@gmail.com>, linux-kernel@vger.kernel.org,
        Andreas Herrmann <andreas.herrmann3@amd.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Suresh Siddha <suresh.b.siddha@intel.com>
Date: Thu, 27 Sep 2012 20:29:44 +0200
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On Thu, 2012-09-27 at 11:19 -0700, Linus Torvalds wrote:
> On Thu, Sep 27, 2012 at 11:05 AM, Borislav Petkov <bp@alien8.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?

Don't forget to run the desktop interactivity benchmarks after you're
done wriggling with this knob... wakeup preemption is important for most
those.