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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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