Messages in this thread | | | Date | Mon, 20 Jan 2014 19:25:33 +0100 | From | Sebastian Reichel <> | Subject | Re: [11/11] system 1: Saving energy using DVFS |
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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 [unhandled content-type:application/pgp-signature] | |