Messages in this thread | | | Date | Mon, 20 Jan 2014 18:03:22 +0000 | From | Catalin Marinas <> | Subject | Re: [11/11] system 1: Saving energy using DVFS |
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On Mon, Jan 20, 2014 at 05:47:45PM +0000, Pavel Machek wrote: > On Mon 2014-01-20 17:17:52, Catalin Marinas wrote: > > On Mon, Jan 20, 2014 at 05:10:29PM +0000, 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: > > > > > > 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. > > > > BTW, the proper way to calculate this is to use the energy rather than > > current x time. This would be J = Ohm * A^2 * s = V^2 / Ohm * s (so the > > impact of the current is even bigger). > > You are claiming that energy is proportional to current squared? > > I stand by numbers. Energy is proportional to values I quoted, > provided constant voltage.
The big advantage of frequency scaling is that you can scale down the voltage, making the power proportional to the voltage squared (or current squared for a constant resistance).
-- Catalin
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