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Date: Thu, 27 Sep 2012 21:24:27 +0200
From: Borislav Petkov <bp@alien8.de>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        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>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
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On Thu, Sep 27, 2012 at 08:29:44PM +0200, Peter Zijlstra 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.

Setting sched_tunable_scaling to SCHED_TUNABLESCALING_LINEAR made
wakeup_granularity go to 4ms:

sched_autogroup_enabled:1
sched_child_runs_first:0
sched_latency_ns:24000000
sched_migration_cost_ns:500000
sched_min_granularity_ns:3000000
sched_nr_migrate:32
sched_rt_period_us:1000000
sched_rt_runtime_us:950000
sched_shares_window_ns:10000000
sched_time_avg_ms:1000
sched_tunable_scaling:2
sched_wakeup_granularity_ns:4000000

pgbench results look good:

tps = 4997.675331 (including connections establishing)
tps = 5003.256870 (excluding connections establishing)

This is still with Ingo's NO_WAKEUP_PREEMPTION patch.

Thanks.

-- 
Regards/Gruss,
Boris.