linux-kernel - Re: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO

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Date:	Tue, 20 Apr 2010 11:24:00 +0200
From:	Thomas Renninger <trenn@...e.de>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Éric Piel <eric.piel@...mplin-utc.net>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, peterz@...radead.org, tglx@...utronix.de,
	davej@...hat.com, cpufreq@...r.kernel.org
Subject: Re: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO

On Monday 19 April 2010 15:43:25 Arjan van de Ven wrote:
> On Mon, 19 Apr 2010 10:29:47 +0200
> Éric Piel <eric.piel@...mplin-utc.net> wrote:
> > > 
> > > The problem and fix are both verified with the "perf timechar" tool.
> > Hi,
> > I don't doubt that keeping the cpu full frequency during IO can
> > improve some specific workloads, however in your log message you
> > don't explain how much we are loosing.
> 
> first of all, it's so bad that people will just turn the whole power
> management off... at which point fixing the really bad bug is actually
> quite a win
Not sure you fix a bug, I expect this was done on purpose.
The ondemand governor disadvantages processes with alternating short CPU
load peaks and idle sequences.
IO bound processes typically show up with such a behavior.

But I follow Eric and agree that if it costs that much, changing
above sounds sane.
Still, I could imagine some people might want to not raise freq on IO bound
process activity, therefore this should get another ondemand param, similar
to ignore_nice_load.

    Thomas
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