Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752102AbaATSMO (ORCPT ); Mon, 20 Jan 2014 13:12:14 -0500 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:43764 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751696AbaATSMJ (ORCPT ); Mon, 20 Jan 2014 13:12:09 -0500 Date: Mon, 20 Jan 2014 19:12:08 +0100 From: Pavel Machek To: Catalin Marinas 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: <20140120181208.GC25439@amd.pavel.ucw.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140120171010.GB29971@arm.com> 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 > > 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. Aha, I noticed the values are now the other way around. [And notice that if user _does_ lock/turn off the screen after the operation, difference between power consumptions is factor of two. People do turn off screens before putting phone back in pocket.] You are right that as long as user does _not_ wait for the computation result, running at low frequency might make sense. That may be true on cellphone so fast that all the actions are "instant". I have yet to see such cellphone. That probably means that staying on low frequency normally and going to high after cpu is busy for 100msec or so is right thing: if cpu is busy for 100msec, it probably means user is waiting for the result. But it depends on the numbers you did not tell us. I'm pretty sure N900 does _not_ have 11% power consuption at 33% performance; I just assumed so for sake of argument. So, really, details are needed. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- 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/