Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752667AbZFVLv0 (ORCPT ); Mon, 22 Jun 2009 07:51:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751694AbZFVLvS (ORCPT ); Mon, 22 Jun 2009 07:51:18 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:45985 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751562AbZFVLvR (ORCPT ); Mon, 22 Jun 2009 07:51:17 -0400 Date: Mon, 22 Jun 2009 13:51:02 +0200 From: Ingo Molnar To: eranian@gmail.com Cc: LKML , Andrew Morton , Thomas Gleixner , Robert Richter , Peter Zijlstra , Paul Mackerras , Andi Kleen , Maynard Johnson , Carl Love , Corey J Ashford , Philip Mucci , Dan Terpstra , perfmon2-devel Subject: Re: I.3 - Multiplexing and system-wide Message-ID: <20090622115102.GD24366@elte.hu> References: <7c86c4470906161042p7fefdb59y10f8ef4275793f0e@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7c86c4470906161042p7fefdb59y10f8ef4275793f0e@mail.gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1548 Lines: 34 > 3/ Multiplexing and system-wide > > Multiplexing is time-based and it is hooked into the timer tick. > At every tick, the kernel tries to schedule another set of event > groups. > > In tickless kernels if a CPU is idle, no timer tick is generated, > therefore no multiplexing occurs. This is incorrect. It's not > because the CPU is idle, that there aren't any interesting PMU > events to measure. Parts of the CPU may still be active, e.g., > caches and buses. And thus, it is expected that multiplexing still > happens. > > You need to hook up the timer source for multiplexing to something > else which is not affected by tickless. You cannot simply disable > tickless during a measurement because you would not be measuring > the system as it actually behaves. There is only a very small difference between inhibiting nohz and waking up the machine using timers on the same interval. Using the scheduler tick to round-robin counters is a first-level approximation we use: it in essence corresponds to multiplexing counters along a cycle cost metric. If there's sufficient interest we can drive the round-robining by the PMU itself: this makes it HZ and dynticks invariant. The simplified multiplexing we offer now certainly works well enough in practice - if there's interest we can expand it. -- 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/