linux-kernel - Re: [11/11] system 1: Saving energy using DVFS

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Date:	Mon, 20 Jan 2014 17:49:26 +0100
From:	Pavel Machek <pavel@....cz>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	peterz@...radead.org, mingo@...nel.org, rjw@...ysocki.net,
	markgross@...gnar.org, vincent.guittot@...aro.org,
	catalin.marinas@....com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [11/11] system 1: Saving energy using DVFS

Hi!

> To save energy, the higher frequencies should be avoided and only used
> when the application performance requirements can not be satisfied
> otherwise (e.g. spread tasks across more cpus if possible).

I argue this is untrue for any task where user waits for its
completion with screen on. (And that's quite important subset).

Lets take Nokia n900 as an example. 

(source http://wiki.maemo.org/N900_Hardware_Power_Consumption)

Sleeping CPU: 2mA
Screen on: 230mA
CPU loaded: 250mA

Now, lets believe your numbers and pretend system can operate at 33%
of speed with 11% power consumption.

Lets take task that takes 10 seconds on max frequency:

      ~ 10s * 470mA     	     	    = 4700mAs

You suggest running at 33% speed, instead; that means 30 seconds on
low requency.

CPU on low: 25mA (assumed).

     ~ 30s * 255mA			    = 7650mAs

Hmm. So race to idle is good thing on Intel machines, and it is good
thing on ARM design I have access to.

And you even acknowledge it here, right:

> When considering the total system power it may save energy in some
> scenarios by running the cpu faster to allow other power hungry parts of
> the system to be shut down faster. However, this is highly platform and
> application dependent.

									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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