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

[Catalin Marinas <catalin.marinas@arm.com>]

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