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

From: Pavel Machek
Date: Mon Jan 20 2014 - 11:49:33 EST


> 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.


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.


(cesky, pictures)
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