Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765011AbYFKRpV (ORCPT ); Wed, 11 Jun 2008 13:45:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763883AbYFKRpE (ORCPT ); Wed, 11 Jun 2008 13:45:04 -0400 Received: from casper.infradead.org ([85.118.1.10]:46272 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763656AbYFKRpB (ORCPT ); Wed, 11 Jun 2008 13:45:01 -0400 Date: Wed, 11 Jun 2008 10:44:46 -0700 From: Arjan van de Ven To: markh@compro.net Cc: "linux-os (Dick Johnson)" , Jan Engelhardt , "Kok, Auke" , "solsTiCe d'Hiver" , linux-kernel@vger.kernel.org Subject: Re: PROBLEM: no cpu MHz in /proc/cpuinfo on 2.6.25.4-rt6 Message-ID: <20080611104446.3b0ce2d9@infradead.org> In-Reply-To: <4850046C.4070602@compro.net> References: <200806102152.23286.solstice.dhiver@gmail.com> <20080610131314.5f7069e6@infradead.org> <484EE71A.40002@intel.com> <20080611063639.5e1eea5b@infradead.org> <20080611065816.6cff290d@infradead.org> <20080611083644.32afc823@infradead.org> <4850046C.4070602@compro.net> Organization: Intel X-Mailer: Claws Mail 3.3.1 (GTK+ 2.12.10; i386-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1996 Lines: 57 On Wed, 11 Jun 2008 12:59:24 -0400 Mark Hounschell wrote: > Arjan van de Ven wrote: > > On Wed, 11 Jun 2008 10:14:50 -0400 > > "linux-os (Dick Johnson)" wrote: > > > >> I use CPU frequency to scale the output of rdtsc when timing > >> routines being optimized. > > > > Ok this is broken in various really bad ways. > > > > 1) CPU frequency is very fluctuating, it changes in principle every > > millisecond or more > > I'm curious. What do you mean by "in principle"? And why will it > change? ondemand governor will change the cpu frequency dynamically all the time. the cpu itself has a dynamic range in which it operates (at least on cpus that support Intel Dynamic Acceleration technology, IDA) > > > 2) the rdtsc "frequency" is conceptually unrelated to cpu > > frequency. In fact, you'll be hard-pressed to buy a system today > > where this relationship works.... > > > > And what do you mean by "conceptually unrelated to cpu frequency"? > Is it not the clock freq that is driving the cpu and the freq at which > the tsc is incremented? no it is not that... at all. the tsc comes from an entirely different clock, and on anything you can buy today from AMD or Intel (or the last year for that matter), it's fixed frequency (except in idle) irrespective of the frequency the CPU is operating at! it's a "time stamp counter" not a "cpu cycle counter". You can run instructions faster than the tsc increment or slower. Or sometimes at the same rate. Which it is depends on what cpufreq/ondemand are doing and how active IDA is. -- If you want to reach me at my work email, use arjan@linux.intel.com For development, discussion and tips for power savings, visit http://www.lesswatts.org -- 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/