From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754545Ab0CECeQ (ORCPT ); Thu, 4 Mar 2010 21:34:16 -0500 Received: from smtp-out.google.com ([216.239.33.17]:22111 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754473Ab0CECeP (ORCPT ); Thu, 4 Mar 2010 21:34:15 -0500 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:date:message-id:subject:from:to: cc:content-type:x-system-of-record; b=fic7A0KWzGtMkgJRG6L5y2o04U/LMjPR1KQHQfokLNHD292Iax21f63xbWHOAOvW8 9arYhO1QWy16tzvt4T1nQ== MIME-Version: 1.0 In-Reply-To: <20100304211332.GA3643@elte.hu> References: <1267575705-10054-1-git-send-email-eranian@google.com> <20100304113212.GG21977@elte.hu> <20100304211332.GA3643@elte.hu> Date: Thu, 4 Mar 2010 18:34:09 -0800 Message-ID: Subject: Re: [PATCH] perf_events: add sampling period randomization support (v2) From: Stephane Eranian To: Ingo Molnar 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 Content-Type: text/plain; charset=UTF-8 X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). > 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? > We know how long each sample takes so the result is precise, via > PERF_SAMPLE_PERIOD. > I understand that. > -c is legacy in essence. The default is auto-freq and i doubt anyone uses -c > anymore. > The -c option may be more convenient when not counting events correlated with time.