Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752389AbaATSD4 (ORCPT ); Mon, 20 Jan 2014 13:03:56 -0500 Received: from fw-tnat.austin.arm.com ([217.140.110.23]:28006 "EHLO collaborate-mta1.arm.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751087AbaATSDy (ORCPT ); Mon, 20 Jan 2014 13:03:54 -0500 Date: Mon, 20 Jan 2014 18:03:22 +0000 From: Catalin Marinas To: Pavel Machek Cc: Morten Rasmussen , "peterz@infradead.org" , "mingo@kernel.org" , "rjw@rjwysocki.net" , "markgross@thegnar.org" , "vincent.guittot@linaro.org" , "linux-pm@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [11/11] system 1: Saving energy using DVFS Message-ID: <20140120180322.GI29971@arm.com> References: <1389111587-5923-1-git-send-email-morten.rasmussen@arm.com> <1389111587-5923-12-git-send-email-morten.rasmussen@arm.com> <20140120164926.GB23051@amd.pavel.ucw.cz> <20140120171010.GB29971@arm.com> <20140120171752.GC29971@arm.com> <20140120174745.GA25439@amd.pavel.ucw.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140120174745.GA25439@amd.pavel.ucw.cz> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 20, 2014 at 05:47:45PM +0000, Pavel Machek wrote: > On Mon 2014-01-20 17:17:52, Catalin Marinas wrote: > > On Mon, Jan 20, 2014 at 05:10:29PM +0000, Catalin Marinas wrote: > > > On Mon, Jan 20, 2014 at 04:49:26PM +0000, Pavel Machek wrote: > > > > > To save energy, the higher frequencies should be avoided and only used > > > > > when the application performance requirements can not be satisfied > > > > > otherwise (e.g. spread tasks across more cpus if possible). > > > > > > > > I argue this is untrue for any task where user waits for its > > > > completion with screen on. (And that's quite important subset). > > > > > > > > Lets take Nokia n900 as an example. > > > > > > > > (source http://wiki.maemo.org/N900_Hardware_Power_Consumption) > > > > > > > > Sleeping CPU: 2mA > > > > Screen on: 230mA > > > > CPU loaded: 250mA > > > > > > > > Now, lets believe your numbers and pretend system can operate at 33% > > > > of speed with 11% power consumption. > > > > > > > > Lets take task that takes 10 seconds on max frequency: > > > > > > > > ~ 10s * 470mA = 4700mAs > > > > > > > > You suggest running at 33% speed, instead; that means 30 seconds on > > > > low requency. > > > > > > > > CPU on low: 25mA (assumed). > > > > > > > > ~ 30s * 255mA = 7650mAs > > > > > > > > Hmm. So race to idle is good thing on Intel machines, and it is good > > > > thing on ARM design I have access to. > > > > > > Race to idle doesn't mean that the screen goes off as well. Let's say > > > the screen stays on for 1 min and the CPU needs to be running for 10s > > > over this minute, in the first case you have: > > > > > > 10s & 250mA + 60s * 230mA = 16300mAs > > > > > > in the second case you have: > > > > > > 30s * 25mA + 60s * 230mA = 14550mAs > > > > > > That's a 1750mAs difference. There are of course other parts drawing > > > current but simple things like the above really make a difference in the > > > mobile space, both in terms of battery and thermal budget. > > > > BTW, the proper way to calculate this is to use the energy rather than > > current x time. This would be J = Ohm * A^2 * s = V^2 / Ohm * s (so the > > impact of the current is even bigger). > > You are claiming that energy is proportional to current squared? > > I stand by numbers. Energy is proportional to values I quoted, > provided constant voltage. The big advantage of frequency scaling is that you can scale down the voltage, making the power proportional to the voltage squared (or current squared for a constant resistance). -- Catalin -- 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/