From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932476AbVLER2N (ORCPT ); Mon, 5 Dec 2005 12:28:13 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932478AbVLER2M (ORCPT ); Mon, 5 Dec 2005 12:28:12 -0500 Received: from mx1.redhat.com ([66.187.233.31]:8678 "EHLO mx1.redhat.com") by vger.kernel.org with ESMTP id S932477AbVLER2K (ORCPT ); Mon, 5 Dec 2005 12:28:10 -0500 Date: Mon, 5 Dec 2005 12:27:54 -0500 From: Dave Jones To: Avi Kivity Cc: Lee Revell , Andi Kleen , Venkatesh Pallipadi , Andrew Morton , cpufreq , linux-kernel Subject: Re: [PATCH] CPU frequency display in /proc/cpuinfo Message-ID: <20051205172754.GD12664@redhat.com> Mail-Followup-To: Dave Jones , Avi Kivity , Lee Revell , Andi Kleen , Venkatesh Pallipadi , Andrew Morton , cpufreq , linux-kernel References: <20051202181927.GD9766@wotan.suse.de> <20051202104320.A5234@unix-os.sc.intel.com> <20051204164335.GB32492@isilmar.linta.de> <20051204183239.GE14247@wotan.suse.de> <1133725767.19768.12.camel@mindpipe> <20051205011611.GA12664@redhat.com> <43946ACE.9040405@argo.co.il> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <43946ACE.9040405@argo.co.il> User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 05, 2005 at 06:29:02PM +0200, Avi Kivity wrote: > Dave Jones wrote: > > >I can't think of a single valid reason why a program would want > >to know the MHz rating of a CPU. Given that it's a) approximate, > >b) subject to change due to power management, c) completely nonsensical > >across CPU vendors, and d) only one of many variables regarding CPU > >performance, any program that bases any decision on the values found > >by parsing that field of /proc/cpuinfo is utterly broken beyond belief. > > > > > Sometimes you need extremely low overhead time measurements, which need > not be too accurate. One way to do this is to dump rdtsc measurements > into some array, and later scale it using the cpu frequency. > > I've done exactly this. The processes were pinned to their processors, > and there was no frequency scaling in effect. It worked very well. That code will break as soon as it's run on a CPU that uses P-states. You can't "just scale" the value, there are other factors at work. Dave