From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dominik Brodowski Subject: Re: Status of X86_P4_CLOCKMOD? Date: Sat, 25 Feb 2006 14:28:20 +0100 Message-ID: <20060225132820.GA13413@isilmar.linta.de> References: <20060214152218.GI10701@stusta.de> <20060223204110.GE6213@redhat.com> <20060225015722.GC8132@linuxtv.org> <200602250527.03493.ak@suse.de> <20060225125326.GJ3674@stusta.de> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20060225125326.GJ3674@stusta.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Adrian Bunk Cc: Andi Kleen , Johannes Stezenbach , Dave Jones , Dmitry Torokhov , davej@codemonkey.org.uk, Zwane Mwaikambo , Samuel Masham , Jan Engelhardt , linux-kernel@vger.kernel.org, Andrew Morton , cpufreq@lists.linux.org.uk On Sat, Feb 25, 2006 at 01:53:26PM +0100, Adrian Bunk wrote: > > > Doesn't less heat imply less power consumption? > > > > Not in this case no. > >... > > Sorry for the dumb question, but how could this work physically? > > If a computer produces less heat with the same power consumption, what > happens with the other energy? No. Let's do the math (again), and (again) for the actual values of an Intel Pentium(R) M Processor, 1400 MHz @ 1.484 V, even though the same rules of physics, logic and mathematics apply to _all_ processors. Power consumption in idle state C2 (Stop-Grant state) 7.3 W Power consumption when "skipping instructions" because of throttling (Stop-Grant state) 7.3 W Power consumption when doing work 22.0 W This means that if the processor idle percentage is _larger_ than (1 - throttling rate), throttling has no effect at all. Now, let's assume there is some work for the CPU to do which keeps it busy for one second @ 1.4 GHz. How much energy is needed to get this work done? 0% throttling: 22 Ws (1s) 25% throttling: 24 Ws (1.3s) 50% throttling: 29 Ws (2s) 75% throttling: 44 Ws (4s) Now let's also assume there is nothing else to do during a span of four seconds: then, independent of the throttling setting, the CPU power consumption is 44 Ws for these four seconds. However: for the 75% throttling state, the CPU only produces 11 W of heat _all the time_ -- this means, the fan or air conditioning must only consider 11 W. For 0%, the CPU may produce 44 W of heat in a second -- and to cool that sufficiently, the fan _may_ need to run faster, which consumes more energy than is saved by only having to cool 7.3 W (instead of 11W) the other three seconds. So: P4-clockmod style throttling only makes sense if either a) the idle handler does not enter the Stop-Grant state (C2) efficiently, or b) the load varies significantly over time in a manner which has effect on the fan, and where the latency induced by throttling doesn't matter. Dominik