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

[Sebastian Reichel <sre@ring0.de>]

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On Mon, Jan 20, 2014 at 06:54:32PM +0100, Pavel Machek wrote:
> 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.

So what about using the display status information for power
management? Basically always using the lowest frequency should be ok
on phones if the display is disabled?

-- Sebastian

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