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    SubjectRe: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.
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    On Tue, 2009-06-16 at 13:37 +0530, Vaidyanathan Srinivasan wrote:
    > * Andrew Morton <akpm@linux-foundation.org> [2009-06-15 23:23:18]:
    >
    > > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@in.ibm.com> wrote:
    > >
    > > > Currently on a ppc64 box with 16 CPUs, the time taken for
    > > > a individual cpu-hotplug operation is as follows.
    > > >
    > > > # time echo 0 > /sys/devices/system/cpu/cpu2/online
    > > > real 0m0.025s
    > > > user 0m0.000s
    > > > sys 0m0.002s
    > > >
    > > > # time echo 1 > /sys/devices/system/cpu/cpu2/online
    > > > real 0m0.021s
    > > > user 0m0.000s
    > > > sys 0m0.000s
    > >
    > > Surprised. Do people really online and offline CPUs frequently enough
    > > for this to be a problem?
    >
    > Certainly not for hardware faults or hardware replacement, but
    > cpu-hotplug interface is useful for changing system configuration to
    > meet different objectives like
    >
    > * Reduce system capacity to reduce average power and reduce heat
    >
    > * Increasing number of cores and threads in a CPU package is leading
    > to multiple cpu offline/online operations for any perceivable effect
    >
    > * Dynamically change CPU configurations in virtualized environments

    I tend to agree with Andrew, if any of those things are done frequent
    enough that the hotplug performance matter you're doing something mighty
    odd.



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