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    SubjectRe: CPU offline but power consumption increased?
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    On Saturday 07 April 2007 19:38, Andi Kleen wrote:
    > "Andika Triwidada" <andika@gmail.com> writes:
    >
    > [cc linux-acpi]
    >
    > > Question: is that normal? I thought power consumption will be
    > > automatically reduced if one core offlined.

    Known? Yes.
    What people would expect? No.

    http://bugzilla.kernel.org/show_bug.cgi?id=5471

    > The current cpu offline essentially just runs a special idle loop.
    > The standard idle loop is even a bit more aggressive on some systems
    > because it knows about the deeper ACPI sleep modi.
    >
    > There are also dependencies between cores because current CPUs
    > have shared power planes between cores.
    >
    > I suppose in the future when a whole socket goes off line one could
    > implement special code to turn off the CPU further. But it likely
    > won't work on older hardware.

    Speaking for all Intel hardware implemented from pre-history until now,
    deep C-states is the best you can do, and there is no special offline
    mode to save more power.

    If you really want to not use a core and the above bug isn't fixed in linux,
    you can use maxcpus=1 to never bring the other core on-line in the first place,
    and if the BIOS is implemented properly, the core will be spinning in the
    deepest available C-state. Of course, it would probably be more interesting
    to simply leave the core on-line and let it go idle...

    -Len
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