[PATCH v2 0/2] cpu: pseries: Offline state framework.

Arjan van de Ven arjan at infradead.org
Thu Sep 24 21:41:23 EST 2009

On Thu, 24 Sep 2009 13:33:07 +0200
Peter Zijlstra <a.p.zijlstra at chello.nl> wrote:

> On Thu, 2009-09-24 at 18:38 +1000, Benjamin Herrenschmidt wrote:
> > On Thu, 2009-09-24 at 09:51 +0200, Peter Zijlstra wrote:
> > > > I don't quite follow your logic here. This is useful for more
> > > > than just hypervisors. For example, take the HV out of the
> > > > picture for a moment and imagine that the HW has the ability to
> > > > offline CPU in various power levels, with varying latencies to
> > > > bring them back.
> > > 
> > > cpu-hotplug is an utter slow path, anybody saying latency and
> > > hotplug in the same sentence doesn't seem to grasp either or both
> > > concepts.
> > 
> > Let's forget about latency then. Let's imagine I want to set a CPU
> > offline to save power, vs. setting it offline -and- opening the back
> > door of the machine to actually physically replace it :-)
> If the hardware is capable of physical hotplug, then surely powering
> the socket down saves most power and is the preferred mode?

btw just to take away a perception that generally powering down sockets
help; it does not help for all cpus. Some cpus are so efficient in idle
that the incremental gain one would get by "offlining" a core is just
not worth it
(in fact, in x86, it's the same thing)

I obviously can't speak for p-series cpus, just wanted to point out
that there is no universal truth about "offlining saves power".

Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

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