Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752590AbZFWFQq (ORCPT ); Tue, 23 Jun 2009 01:16:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751198AbZFWFQi (ORCPT ); Tue, 23 Jun 2009 01:16:38 -0400 Received: from bilbo.ozlabs.org ([203.10.76.25]:34612 "EHLO bilbo.ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080AbZFWFQi (ORCPT ); Tue, 23 Jun 2009 01:16:38 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <19008.25905.550815.222975@drongo.ozlabs.ibm.com> Date: Tue, 23 Jun 2009 15:16:33 +1000 From: Paul Mackerras To: Ingo Molnar Cc: eranian@gmail.com, LKML , Andrew Morton , Thomas Gleixner , Robert Richter , Peter Zijlstra , Andi Kleen , Maynard Johnson , Carl Love , Corey J Ashford , Philip Mucci , Dan Terpstra , perfmon2-devel Subject: Re: I.2 - Grouping In-Reply-To: <20090622115017.GC24366@elte.hu> References: <7c86c4470906161042p7fefdb59y10f8ef4275793f0e@mail.gmail.com> <20090622115017.GC24366@elte.hu> X-Mailer: VM 8.0.12 under 22.3.1 (powerpc-unknown-linux-gnu) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1888 Lines: 41 Ingo Molnar writes: > > 2/ Grouping > > > > By design, an event can only be part of one group at a time. To clarify this statement of Stephane's, a _counter_ can only be in one group. You can have multiple counters counting the same _event_ and those counters can (obviously) be in different groups. > > Events in a group are guaranteed to be active on the PMU at the > > same time. That means a group cannot have more events than there > > are available counters on the PMU. Tools may want to know the > > number of counters available in order to group their events > > accordingly, such that reliable ratios could be computed. It seems > > the only way to know this is by trial and error. This is not > > practical. > > Groups are there to support heavily constrained PMUs, and for them > this is the only way, as there is no simple linear expression for > how many counters one can load on the PMU. That's not the only reason for having groups, or even the main reason IMO. The main reason for having groups is to provide a way to ask the scheduler to ensure that two or more counters are always scheduled together, so that you can meaningfully do arithmetic operations on the counter values that would be sensitive to the statistical noise introduced by the scheduling, such as ratios and differences. In other words, grouping is there because we don't guarantee to have all counters scheduled onto the PMU whenever possible. Heavily constrained PMUs increase the need for scheduling, but even if counters are completely orthogonal there are only a fixed number of them so we still need to schedule counters at some point. Paul. -- 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/