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

From: Pavel Machek
Date: Mon Jan 20 2014 - 13:12:27 EST

> > 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.
(cesky, pictures)
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