linux-kernel - Re: [PATCH] cpufreq: ondemand: Introduces stepped frequency increase

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Message-ID: <20090708174741.GA23487@srcf.ucam.org>
Date:	Wed, 8 Jul 2009 18:47:41 +0100
From:	Matthew Garrett <mjg@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	cpufreq@...r.kernel.org
Subject: Re: [PATCH] cpufreq: ondemand: Introduces stepped frequency
	increase

On Wed, Jul 08, 2009 at 07:41:23PM +0200, Corrado Zoccolo wrote:
> Hi Matthew,
> > Is this a measured powersaving? The ondemand model is based on the
> > assumption that the idle state is disproportionately lower in power than
> > any running state, and therefore it's more sensible to run flat out for
> > short periods of time than run at half speed for longer. Is this
> > inherently flawed, or is it an artifact of differences in your processor
> > design?
> 
> The flawed assumption is that running at doubled frequency halves the
> completion time.
> On cpus that can change the core speed without impacting the
> memory-cache bandwidth
> (i.e. the Pentium M), workloads that access lot of memory go at the
> same speed at
> maximum and minimum frequency.
> Now I see new CPUs that can flush their cache during deep idle states (Atoms),
> this aggravates the aforementioned problem, rendering the high
> frequency state much less appetible.

Do you have numbers to support this? What effect does the ramping up 
have on user-visible latency?

-- 
Matthew Garrett | mjg59@...f.ucam.org
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