Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756697AbYLEMNe (ORCPT ); Fri, 5 Dec 2008 07:13:34 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751667AbYLEMNZ (ORCPT ); Fri, 5 Dec 2008 07:13:25 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:35306 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750895AbYLEMNY (ORCPT ); Fri, 5 Dec 2008 07:13:24 -0500 Date: Fri, 5 Dec 2008 13:13:08 +0100 From: Ingo Molnar To: David Miller Cc: a.p.zijlstra@chello.nl, paulus@samba.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, akpm@linux-foundation.org, eranian@googlemail.com, dada1@cosmosbay.com, robert.richter@amd.com, arjan@infradead.org, hpa@zytor.com, rostedt@goodmis.org Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux Message-ID: <20081205121308.GB28662@elte.hu> References: <20081205082431.GD2030@elte.hu> <20081205.002701.172921476.davem@davemloft.net> <20081205084233.GE2030@elte.hu> <20081205.004922.244597525.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20081205.004922.244597525.davem@davemloft.net> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean 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=3.2.3 -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: 952 Lines: 25 * David Miller wrote: > From: Ingo Molnar > Date: Fri, 5 Dec 2008 09:42:33 +0100 > > > Please let me repeat: it's a _fundamental_ thesis of performance > > instrumentation to not disturb the monitored context. Your insistence > > on _stopping_ the monitored task breaks that fundamental axiom! > > This is only a problem if you make your measurement quantums too small. But if you make the measurement long enough - say we make it 100,000 usecs, then 0.2 usecs of delay between two read()s is insignificant statistically, right? It's a 1:500,000 ratio. Scheduling out a task and back is far more drastic of an effect than any new events in 0.2 usecs. Ingo -- 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/