Messages in this thread | | | Date | Mon, 20 Jan 2014 19:12:08 +0100 | From | Pavel Machek <> | Subject | Re: [11/11] system 1: Saving energy using DVFS |
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> > 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: > > 10s & 250mA + 60s * 230mA = 16300mAs > > in the second case you have: > > 30s * 25mA + 60s * 230mA = 14550mAs > > That's a 1750mAs difference. There are of course other parts drawing > current but simple things like the above really make a difference in the > mobile space, both in terms of battery and thermal budget.
Aha, I noticed the values are now the other way around. [And notice that if user _does_ lock/turn off the screen after the operation, difference between power consumptions is factor of two. People do turn off screens before putting phone back in pocket.]
You are right that as long as user does _not_ wait for the computation result, running at low frequency might make sense. That may be true on cellphone so fast that all the actions are "instant". I have yet to see such cellphone. That probably means that staying on low frequency normally and going to high after cpu is busy for 100msec or so is right thing: if cpu is busy for 100msec, it probably means user is waiting for the result.
But it depends on the numbers you did not tell us. I'm pretty sure N900 does _not_ have 11% power consuption at 33% performance; I just assumed so for sake of argument.
So, really, details are needed. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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