Messages in this thread | | | Date | Mon, 20 Jan 2014 17:49:26 +0100 | From | Pavel Machek <> | Subject | Re: [11/11] system 1: Saving energy using DVFS |
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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
-- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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