Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933351AbcCIQjo (ORCPT ); Wed, 9 Mar 2016 11:39:44 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:39385 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753726AbcCIQji (ORCPT ); Wed, 9 Mar 2016 11:39:38 -0500 Date: Wed, 9 Mar 2016 17:39:30 +0100 From: Peter Zijlstra To: "Rafael J. Wysocki" Cc: "Rafael J. Wysocki" , Steve Muckle , Vincent Guittot , Linux PM list , Juri Lelli , ACPI Devel Maling List , Linux Kernel Mailing List , Srinivas Pandruvada , Viresh Kumar , Michael Turquette , Ingo Molnar Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data Message-ID: <20160309163930.GP6356@twins.programming.kicks-ass.net> References: <2495375.dFbdlAZmA6@vostro.rjw.lan> <56D8AEB7.2050100@linaro.org> <36459679.vzZnOsAVeg@vostro.rjw.lan> <20160308112759.GF6356@twins.programming.kicks-ass.net> <20160308192640.GD6344@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3210 Lines: 107 On Tue, Mar 08, 2016 at 09:05:50PM +0100, Rafael J. Wysocki wrote: > >> This means that on platforms where the utilization is frequency > >> invariant we should use > >> > >> next_freq = a * x > >> > >> (where x is given by (2) above) and for platforms where the > >> utilization is not frequency invariant > >> > >> next_freq = a * x * current_freq / max_freq > >> > >> and all boils down to finding a. > > > > Right. > > However, that doesn't seem to be in agreement with the Steve's results > posted earlier in this thread. I could not make anything of those numbers. > Also theoretically, with frequency invariant, the only way you can get > to 100% utilization is by running at the max frequency, so the closer > to 100% you get, the faster you need to run to get any further. That > indicates nonlinear to me. I'm not seeing that, you get that by using a > 1. No need for non-linear. > >> Now, it seems reasonable for a to be something like (1 + 1/n) * > >> max_freq, so for non-frequency invariant we get > >> > >> nex_freq = (1 + 1/n) * current_freq * x > > > > This seems like a big leap; where does: > > > > (1 + 1/n) * max_freq > > > > come from? And what is 'n'? > a = max_freq gives next_freq = max_freq for x = 1, next_freq = a * x * current_freq / max_freq [ a := max_freq, x := 1 ] -> = max_freq * 1 * current_freq / max_freq = current_freq != max_freq But I think I see what you're saying; because at x = 1, current_frequency must be max_frequency. Per your earlier point. > but with that choice of a you may never get to x = 1 with frequency > invariant because of the feedback effect mentioned above, so the 1/n > produces the extra boost needed for that (n is a positive integer). OK, so that gets us: a = (1 + 1/n) ; n > 0 [ I would not have chosen (1 + 1/n), but lets stick to that ] So for n = 4 that gets you: a = 1.25, which effectively gets you an 80% utilization tipping point. That is, 1.25 * .8 = 1, iow. you'll pick the next frequency (assuming RELATION_L like selection). Together this gets you: next_freq = (1 + 1/n) * max_freq * x * current_freq / max_freq = (1 + 1/n) * x * current_freq Again, with n = 4, x > .8 will result in a next_freq > current_freq, and hence (RELATION_L) pick a higher one. > Quite frankly, to me it looks like linear really is a better > approximation for "raw" utilization. That is, for frequency invariant > x we should take: > > next_freq = a * x * max_freq / current_freq (its very confusing how you use 'x' for both invariant and non-invariant). That doesn't make sense, remember: util = \Sum_i u_i * freq_i / max_freq (1) Which for systems where freq_i is constant reduces to: util = util_raw * current_freq / max_freq (2) But you cannot reverse this. IOW you cannot try and divide out current_freq on a frequency invariant metric. So going by: next_freq = (1 + 1/n) * max_freq * util (3) if we substitute (2) into (3) we get: = (1 + 1/n) * max_freq * util_raw * current_freq / max_freq = (1 + 1/n) * current_freq * util_raw (4) Which gets you two formula with the same general behaviour. As (2) is the only approximation of (1) we can make.