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    SubjectRe: [PATCH v2 0/2] cpu: pseries: Offline state framework.
    On Thu, 24 Sep 2009 13:33:07 +0200
    Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

    > On Thu, 2009-09-24 at 18:38 +1000, Benjamin Herrenschmidt wrote:
    > > On Thu, 2009-09-24 at 09:51 +0200, Peter Zijlstra wrote:
    > > > > I don't quite follow your logic here. This is useful for more
    > > > > than just hypervisors. For example, take the HV out of the
    > > > > picture for a moment and imagine that the HW has the ability to
    > > > > offline CPU in various power levels, with varying latencies to
    > > > > bring them back.
    > > >
    > > > cpu-hotplug is an utter slow path, anybody saying latency and
    > > > hotplug in the same sentence doesn't seem to grasp either or both
    > > > concepts.
    > >
    > > Let's forget about latency then. Let's imagine I want to set a CPU
    > > offline to save power, vs. setting it offline -and- opening the back
    > > door of the machine to actually physically replace it :-)
    >
    > If the hardware is capable of physical hotplug, then surely powering
    > the socket down saves most power and is the preferred mode?

    btw just to take away a perception that generally powering down sockets
    help; it does not help for all cpus. Some cpus are so efficient in idle
    that the incremental gain one would get by "offlining" a core is just
    not worth it
    (in fact, in x86, it's the same thing)

    I obviously can't speak for p-series cpus, just wanted to point out
    that there is no universal truth about "offlining saves power".

    --
    Arjan van de Ven Intel Open Source Technology Centre
    For development, discussion and tips for power savings,
    visit http://www.lesswatts.org


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