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:

quoted

quoted

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).

-- 
Catalin