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

Hi!

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.

And you even acknowledge it here, right:

When considering the total system power it may save energy in some
scenarios by running the cpu faster to allow other power hungry parts of
the system to be shut down faster. However, this is highly platform and
application dependent.

									Pavel

-- 
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