Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.
From: Ingo Molnar
Date: Wed Jun 17 2009 - 11:08:02 EST
* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Jun 17, 2009 at 09:32:57AM +0200, Peter Zijlstra wrote:
> > On Tue, 2009-06-16 at 13:37 +0530, Vaidyanathan Srinivasan wrote:
> > > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [2009-06-15 23:23:18]:
> > >
> > > > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@xxxxxxxxxx> 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.
>
> Boot speedup?
Also, if it brings more attention (and more stability and more
bugfixes) to CPU hotplug that's only good.
Ingo
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