From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757591Ab0CKLwv (ORCPT ); Thu, 11 Mar 2010 06:52:51 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:59295 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757553Ab0CKLwu (ORCPT ); Thu, 11 Mar 2010 06:52:50 -0500 Date: Thu, 11 Mar 2010 12:52:14 +0100 From: Ingo Molnar To: Stephane Eranian Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, paulus@samba.org, fweisbec@gmail.com, perfmon2-devel@lists.sf.net, robert.richter@amd.com, eranian@gmail.com, davem@davemloft.net, Arnaldo Carvalho de Melo Subject: Re: [PATCH] perf_events: add sampling period randomization support (v2) Message-ID: <20100311115214.GD31354@elte.hu> References: <1267575705-10054-1-git-send-email-eranian@google.com> <20100304113212.GG21977@elte.hu> <20100304211332.GA3643@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: 0.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=0.0 required=5.9 tests=none autolearn=no SpamAssassin version=3.2.5 _SUMMARY_ Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Stephane Eranian wrote: > On Thu, Mar 4, 2010 at 1:13 PM, Ingo Molnar wrote: > > > > * Stephane Eranian wrote: > > > >> On Thu, Mar 4, 2010 at 3:32 AM, Ingo Molnar wrote: > >> > > >> > * eranian@google.com wrote: > >> > > >> >> This patch adds support for randomizing the sampling period. ??Randomization > >> >> is very useful to mitigate the bias that exists with sampling. The random > >> >> number generator does not need to be sophisticated. This patch uses the > >> >> builtin random32() generator. > >> > > >> >> + ?? ?? if (width > 63 || attr->freq) > >> >> + ?? ?? ?? ?? ?? ?? return -EINVAL; > >> > > >> > Why not for freq counters? Those are semi-randomized already, but it might > >> > make sense to make them 'more' randomized in special circumstances. That would > >> > also allow us to enable the randomization in perf top and perf record, by > >> > default. > >> > > >> > >> What's the goal of freq? > >> Achieve and maintain the target interrupt/rate. > >> In doing so, it has to adjust the period (not randomly). > > > > No, the goal of auto-freq is to keep a steady average rate of sampling. > > rate of samples = rate of interrupts (if period < 32 bits on Intel). What's your point? I corrected your statement which said that the goal of auto-freq was to maintain a target interrupt-rate and as such wouldnt be randomizable. So i said that auto-freq is slightly different from that: it provides a steady _average_ rate, and as such small amounts of randomization 'fuzz' could still be injected - the auto-freq system would auto-correct the effects of that. Think of it as a dynamic steady-state equilibrium with noise injected. If the noise isnt too brutal and the system can adapt, the average sampling rate doesnt change. > > There is no requirement to keep it 'steady' - each sample comes with a > > specific weight. > > > >> Randomization may prevent achieving the rate, or it may slow it down. > >> What's the value add of that? > > > > Why do you assume that the two are incompatible? We can randomize > > auto-freq and still have a perfectly stable average rate. > > What would that buy you compared to what you already have? The same goal as randomization in general: to decrease the chance for sampling artifacts that can occur due to the sampling frequency oscillating together with some internal workload parameter, skewing the sample. Ingo