From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756868Ab2BHLIM (ORCPT ); Wed, 8 Feb 2012 06:08:12 -0500 Received: from mail-lpp01m020-f174.google.com ([209.85.217.174]:47215 "EHLO mail-lpp01m020-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755332Ab2BHLIK (ORCPT ); Wed, 8 Feb 2012 06:08:10 -0500 MIME-Version: 1.0 In-Reply-To: <4F32539D.8030402@linux.vnet.ibm.com> References: <4F3243D4.4080907@linux.vnet.ibm.com> <4F32539D.8030402@linux.vnet.ibm.com> Date: Wed, 8 Feb 2012 12:08:08 +0100 Message-ID: Subject: Re: Use case for PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event From: Stephane Eranian To: Anshuman Khandual Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, mingo@elte.hu X-System-Of-Record: true Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 8, 2012 at 11:51 AM, Anshuman Khandual wrote: > On Wednesday 08 February 2012 03:21 PM, Stephane Eranian wrote: >> On Wed, Feb 8, 2012 at 10:43 AM, Anshuman Khandual >> wrote: >>> Hello Stephane, >>> >>> I was going through the following discussion where we added the >>> new HW generic event PERF_COUNT_HW_REF_CPU_CYCLES. >>> >>> https://lkml.org/lkml/2011/12/10/103 >>> >>> (Sorry, for asking this question bit late) >>> >>> I am trying to understand the use case for this. Would this new event >>> help us in generating (during a perf session) a CPU frequency invariant >>> time metric against which we would plot our other perf event's measurements ? >>> CPU frequency independent time measurement is it's primary purpose ? or we were >>> finding a way to expose the fixed counter 2 which was not getting used before >>> for not having an event encoding. I guess this would help us in finding equivalent >>> PMU events or mechanisms in other architecture / platforms. >>> >> The goal was to expose a cycle event that is not subject to frequency scaling >> nor turbo boost of any sort. An event that could be used to correlate with time. > >> An event that could also be used to compute idle time by comparing its value >> with wall-clock time. > > Why kernel computed idle time is not sufficient ? How much accuracy would it > improve in using PMU event computed idle time over kernel computed idle time. > I was just listing the what you could do with the event. I have not yet tried to measure idle with ref-cycles. > >> >> The fact that on Intel X86 this event is on fixed counter 2 is an >> implementation >> detail. >> >>> -- >>> Anshuman Khandual >>> Linux Technology Centre >>> IBM Systems and Technology Group >>> >> >