Messages in this thread | | | Date | Mon, 20 Jan 2014 17:10:29 +0000 | From | Catalin Marinas <> | Subject | Re: [11/11] system 1: Saving energy using DVFS |
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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:
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.
-- Catalin
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