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    SubjectRe: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO
    On Mon 2010-04-19 17:47:02, Arjan van de Ven wrote:
    > On Mon, 19 Apr 2010 16:29:39 +0100
    > Tvrtko Ursulin <tvrtko.ursulin@sophos.com> wrote:
    >
    > > On Monday 19 Apr 2010 14:46:17 Arjan van de Ven wrote:
    > > > On Mon, 19 Apr 2010 10:09:55 +0100
    > > > > Or in other words, does a pure IO workload benefit from now higher
    > > > > selected frequency?
    > > >
    > > > no.
    > > > Mixed workloads do.
    > > > but pure IO workloads also don't suffer since while idle, the
    > > > voltage goes down anyway.
    > >
    > > You mean that higher frequency does not have effect on power use if
    > > CPU is idle? Is that true for all/most processors?
    >
    > this is true for most processors that I'm aware of.
    > there's exceptions for things like where the idle time is really short,

    Is not that exactly what will happen for 'cat /dev/<usb1>' case?

    Plus I suspect that older cpus are slower at changing voltages, and
    slower at powering down when idle...

    > > How and where in the code and how to enable that behaviour? From my
    > > experiments frequency goes down to minimum as soon as load goes away.
    > > What I was talking about is gradual lowering over a configurable
    > > period. It is not power efficient, but it could be good for latency
    > > in some workloads.
    >
    > it's not even good for that ;-(
    >
    > it's better then to stay high longer... at least on modern machines the
    > inbetween states are pretty much either useless or actually energy
    > hurting compared to the higher state.

    So what about hiding those from ondemand on modern hw?
    Pavel

    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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