Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754023AbYLOOmm (ORCPT ); Mon, 15 Dec 2008 09:42:42 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753217AbYLOOmd (ORCPT ); Mon, 15 Dec 2008 09:42:33 -0500 Received: from mail-bw0-f21.google.com ([209.85.218.21]:52849 "EHLO mail-bw0-f21.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753009AbYLOOmc (ORCPT ); Mon, 15 Dec 2008 09:42:32 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=gamma; h=message-id:date:from:reply-to:to:subject:cc:in-reply-to :mime-version:content-type:content-transfer-encoding :content-disposition:references; b=tmyXtwzylyhwbJtpBGKafe46XjBvw3fZYhqXozXkHEDus7DGBjQiHALjs3tU+N9uC0 5HrHn2qLzzuTvjspn/RDkfCW/C4TYyxaKQ/JnxeNOI6T1BHparnHd5sFVODG2tjYktCH h9yaHerQHj8nji5IHU0IEOk2Hz48aOhUsnGpM= Message-ID: <7c86c4470812150642s5c0faa6ck7d48889ba0c2bf57@mail.gmail.com> Date: Mon, 15 Dec 2008 15:42:30 +0100 From: "stephane eranian" Reply-To: eranian@gmail.com To: "Paul Mackerras" Subject: Re: [patch] Performance Counters for Linux, v3 Cc: "Ingo Molnar" , "Peter Zijlstra" , "Vince Weaver" , linux-kernel@vger.kernel.org, "Thomas Gleixner" , "Andrew Morton" , "Eric Dumazet" , "Robert Richter" , "Arjan van de Veen" , "Peter Anvin" , "David S. Miller" In-Reply-To: <18757.42682.109305.676647@drongo.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <20081211155230.GA4230@elte.hu> <1229070345.12883.12.camel@twins> <7c86c4470812120059s7f8e64a6h91ebeadbf938858d@mail.gmail.com> <1229073834.12883.41.camel@twins> <7c86c4470812120942x607a74f7w9f823adecbd73b85@mail.gmail.com> <7c86c4470812121001i765d663bq6db3080b633a1eef@mail.gmail.com> <20081214231332.GA26942@elte.hu> <18757.42682.109305.676647@drongo.ozlabs.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2685 Lines: 66 Hi, On Mon, Dec 15, 2008 at 1:37 AM, Paul Mackerras wrote: > Ingo Molnar writes: > >> * stephane eranian wrote: >> >> > Hi, >> > >> > Given the level of abstractions you are using for the API, and given >> > your argument that the kernel can do the HW resource scheduling better >> > than anybody else. >> > >> > What happens in the following test case: >> > >> > - 2-way system (cpu0, cpu1) >> > >> > - on cpu0, two processes P1, P2, each self-monitoring and counting event E1. >> > Event E1 can only be measured on counter C1. >> > >> > - on cpu1, there is a cpu-wide session, monitoring event E1, thus using C1 >> > >> > - the scheduler decides to migrate P1 onto CPU1. You now have a >> > conflict on C1. >> > >> > How is this managed? >> >> If there's a single unit of sharable resource [such as an event counter, >> or a physical CPU], then there's just three main possibilities: either >> user 1 gets it all, or user 2 gets it all, or they share it. >> >> We've implemented the essence of these variants, with sharing the resource >> being the sane default, and with the sysadmin also having a configuration >> vector to reserve the resource to himself permanently. (There could be >> more variations of this.) >> >> What is your point? > > Note that Stephane said *counting* event E1. > > One of the important things about counting (as opposed to sampling) is > that it matters whether or not the event is being counted the whole > time or only part of the time. Thus it puts constraints on counter > scheduling and reporting that don't apply for sampling. > Paul is right. > In other words, if I'm counting an event, I want it to be counted all > the time (i.e. whenever the task is executing, for a per-task counter, > or continuously for a per-cpu counter). If that causes conflicts and > the kernel decides not to count the event for part of the time, that > is very much second-best, and I absolutely need to know that that > happened, and also when the kernel started and stopped counting the > event (so I can scale the result to get some idea what the result > would have been if it had been counted the whole time). > That is very true. You cannot multiplex events onto counters without applications knowing. They need to know how long each 'set' has been active. This is needed to scale the results. This is especially true for cpu-wide measurements. -- 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/