Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.