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


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