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

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Message-ID: <20140120175431.GB25439@amd.pavel.ucw.cz>
Date:	Mon, 20 Jan 2014 18:54:32 +0100
From:	Pavel Machek <pavel@....cz>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Morten Rasmussen <Morten.Rasmussen@....com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"markgross@...gnar.org" <markgross@...gnar.org>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [11/11] system 1: Saving energy using DVFS

On Mon 2014-01-20 17:10:29, Catalin Marinas wrote:
> On Mon, Jan 20, 2014 at 04:49:26PM +0000, Pavel Machek wrote:
> > > 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.
> 
> Race to idle doesn't mean that the screen goes off as well. Let's say
> the screen stays on for 1 min and the CPU needs to be running for 10s
> over this minute, in the first case you have:

No, it does not. I just assumed user is continuing to use his
machine. Obviously, waiting 60 seconds with screen on will make the
difference look smaller. But your solution still means user has to
wait longer _and_ you consume more battery doing so.

And this is for any task where user waits for result with screen
on. Like rendering a webpage. Like opening settings screen. Like
installing application.

There are not too many background tasks on a cellphone.

But hey, maybe you are right and running at lowest possible frequency
is right. Please provide concrete numbers like I did.

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