Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755867AbYATTOt (ORCPT ); Sun, 20 Jan 2008 14:14:49 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754875AbYATTOm (ORCPT ); Sun, 20 Jan 2008 14:14:42 -0500 Received: from waste.org ([66.93.16.53]:54775 "EHLO waste.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754848AbYATTOl (ORCPT ); Sun, 20 Jan 2008 14:14:41 -0500 Subject: Re: PROBLEM: Celeron Core From: Matt Mackall To: Robert Hancock Cc: David Newall , Andi Kleen , Chodorenko Michail , Linux Kernel Mailing List In-Reply-To: <479391C9.9020004@shaw.ca> References: <479391C9.9020004@shaw.ca> Content-Type: text/plain Date: Sun, 20 Jan 2008 13:14:17 -0600 Message-Id: <1200856457.13649.29.camel@cinder.waste.org> Mime-Version: 1.0 X-Mailer: Evolution 2.12.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2766 Lines: 59 On Sun, 2008-01-20 at 12:24 -0600, Robert Hancock wrote: > David Newall wrote: > > Andi Kleen wrote: > >>> Isn't it the case that an idle machine will use > >>> less power when throttled than when not? > >>> > >> No that is not the case (not even on old CPUs) > >> > > Then why would it run cooler? What generates the heat when not > > throttled? What stops generating heat when throttled? And you say this > > happens without reducing power consumption? I'm not convinced. I'm a > > long way from that. > > I believe that all throttling does is forcibly halt the CPU on a > particular duty cycle. This will reduce the rate of power consumption, > but reduces the CPU performance by a greater amount (since even at 100% > halted the CPU still consumes power) and so actually reduces performance > per watt. It will spread the heat and power usage produced from a given > workload task out in time (thus its usefulness in limiting CPU > temperature) but will consume more power overall. Your usage of "overall power" here is wrong. Power is an instantaneous quantity (1/s) like velocity, and you are comparing it to energy which is not an instaneous quantity, more like distance. If we throttle the velocity of a car from 100km/h to 50km/h, it'll obviously take longer for it travel a given distance. Now what will it mean when we ask about its "overall velocity" when it reaches its destination? We surely don't mean the distance travelled - that's not a velocity! We can perhaps talk about its average velocity, which will obviously be smaller. > Real CPU clock throttling schemes like SpeedStep, PowerNow, etc. > actually do increase performance per watt when they kick in. That may be true. But the statement "throttling does not reduce power usage" remains false. And the statement "throttling reduces heat production but not power usage" remains physically impossible. It might be true that "throttling increases energy usage per unit of computation relative to no power saving measures at all", but that is not incompatible with "throttling lets you run your laptop on battery longer than no power saving measures at all", which is often what people care about. Voltage/frequency reduction is obviously a much better solution if it's available as reducing voltage reduces power usage quadratically rather than linearly. But beyond the quadratic/linear thing, the concept is the same: use less power and your battery lasts longer. -- Mathematics is the supreme nostalgia of our time. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/