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

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Message-ID: <4e5e476b0907081041g5561a8b2s1bc393809a09b78b@mail.gmail.com>
Date:	Wed, 8 Jul 2009 19:41:23 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Matthew Garrett <mjg@...hat.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

Hi Matthew,

On Wed, Jul 8, 2009 at 6:10 PM, Matthew Garrett<mjg@...hat.com> wrote:
> On Wed, Jul 08, 2009 at 03:56:33PM +0200, Corrado Zoccolo wrote:
>> The patch introduces a new sysfs tunable cpufreq/ondemand/freq_step,
>> as found in conservative governor, to chose the frequency increase step,
>> expressed as percentage (default = 100 is previous behaviour).
>>
>> This allows fine tuning powersaving on mobile CPUs, since smaller steps will allow to:
>> * absorb punctual load spikes
>> * stabilize at the needed frequency, without passing for more power consuming states, and
>
> 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.

Corrado

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



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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