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

[Vaidyanathan Srinivasan]

* 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

Ref:

[1] Saving power by cpu evacuation sched_max_capacity_pct=n
    http://lkml.org/lkml/2009/5/13/173

[2] Make offline cpus to go to deepest idle state using
    http://lkml.org/lkml/2009/5/22/431

[3] cpuset: add new API to change cpuset top group's cpus
    http://lkml.org/lkml/2009/5/19/54        

For getting stuff off a certain CPU, cpu-hotplug framework seems to do
the right thing.  Identifying bottlenecks in the framework can
significantly help other use cases.

--Vaidy

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