Messages in this thread | | | Date | Thu, 24 Sep 2009 13:41:23 +0200 | From | Arjan van de Ven <> | Subject | Re: [PATCH v2 0/2] cpu: pseries: Offline state framework. |
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On Thu, 24 Sep 2009 13:33:07 +0200 Peter Zijlstra <a.p.zijlstra@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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