From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from casper.infradead.org (casper.infradead.org [85.118.1.10]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 69396B7B65 for ; Thu, 24 Sep 2009 21:41:32 +1000 (EST) Date: Thu, 24 Sep 2009 13:41:23 +0200 From: Arjan van de Ven To: Peter Zijlstra Subject: Re: [PATCH v2 0/2] cpu: pseries: Offline state framework. Message-ID: <20090924134123.4acd1adf@infradead.org> In-Reply-To: <1253791987.7695.153.camel@twins> References: <20090828095741.10641.32053.stgit@sofia.in.ibm.com> <1251869611.7547.38.camel@twins> <1253753307.7103.356.camel@pasglop> <1253778667.7695.130.camel@twins> <1253781508.7103.437.camel@pasglop> <1253791987.7695.153.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Cc: Gautham R Shenoy , Venkatesh Pallipadi , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, "Darrick J. Wong" List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 24 Sep 2009 13:33:07 +0200 Peter Zijlstra 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