linux-kernel - Re: Scheduler accounting inflated for io bound processes.

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Message-ID: <20130626093713.GA27385@gmail.com>
Date:	Wed, 26 Jun 2013 11:37:13 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Galbraith <bitbucket@...ine.de>
Cc:	Dave Chiluk <chiluk@...onical.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Scheduler accounting inflated for io bound processes.


* Mike Galbraith <bitbucket@...ine.de> wrote:

> On Tue, 2013-06-25 at 18:01 +0200, Mike Galbraith wrote: 
> > On Thu, 2013-06-20 at 14:46 -0500, Dave Chiluk wrote: 
> > > Running the below testcase shows each process consuming 41-43% of it's
> > > respective cpu while per core idle numbers show 63-65%, a disparity of
> > > roughly 4-8%.  Is this a bug, known behaviour, or consequence of the
> > > process being io bound?
> > 
> > All three I suppose.
> 
> P.S.
> 
> perf top --sort=comm -C 3 -d 5 -F 250 (my tick freq)
> 56.65%    netserver
> 43.35%         pert
> 
> perf top --sort=comm -C 3 -d 5
> 67.16%  netserver
> 32.84%       pert
> 
> If you sample a high freq signal (netperf TCP_RR) at low freq (tick),
> then try to reproduce the original signal, (very familiar) distortion
> results.  Perf doesn't even care about softirq yada yada, so seems it's
> a pure sample rate thing.

Would be very nice to randomize the sampling rate, by randomizing the 
intervals within a 1% range or so - perf tooling will probably recognize 
the different weights.

Thanks,

	Ingo
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