From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753222AbaHEOcI (ORCPT ); Tue, 5 Aug 2014 10:32:08 -0400 Received: from mail.kernel.org ([198.145.19.201]:41845 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752133AbaHEOcF (ORCPT ); Tue, 5 Aug 2014 10:32:05 -0400 Date: Tue, 5 Aug 2014 11:31:59 -0300 From: Arnaldo Carvalho de Melo To: Andi Kleen Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Andi Kleen , Peter Zijlstra , Stephane Eranian , Thomas Gleixner , Jiri Olsa Subject: Re: [PATCH 05/33] perf record: Allow the user to disable time stamps Message-ID: <20140805143159.GI13375@kernel.org> References: <1407169064-25625-1-git-send-email-acme@kernel.org> <1407169064-25625-6-git-send-email-acme@kernel.org> <20140805060856.GA9453@gmail.com> <20140805141733.GH5803@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140805141733.GH5803@tassilo.jf.intel.com> X-Url: http://acmel.wordpress.com User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Em Tue, Aug 05, 2014 at 07:17:33AM -0700, Andi Kleen escreveu: > > > Time stamps are always implicitely enabled for record currently. The > > > old --time/-T option is a nop. > > > Allow the user to disable timestamps by using --no-time > > > This can cause some minor misaccounting (by missing mmaps), but > > > significantly lowers the size of perf.data > > I'm not any big change in size: > > -rw------- 1 mingo mingo 384768 Aug 5 08:01 perf.data.timestamps > > -rw------- 1 mingo mingo 336952 Aug 5 08:00 perf.data.notimestamps > It will depend on your workload. What period did you use > (or did it automatically use) and what kind of > workload was it? > The smaller the period, the higher the benefit. > There are some classes of workloads where using a smaller > period is especially beneficial, essentially anything > with lots of small events, instead of long loops. > You also get a higher benefit if you use -c or -F, because > without that each sample is smaller (no period reported) > You also get higher benefit for longer traces, and traces > that do not start a lot of programs, as those > tend to be dominated by MMAP events and other overhead. I guess it would have been great if you had put the above paragraphs in the documentation for 'perf record' :-\ - Arnaldo > > So either remove the --time option altogether, or fix its > > 'misaccounting' so that the profile can be relied on. > I don't know how to fix it. Do you? > I guess one possible way to mitigate would be to lower the perf > buffer sizes, then the worst case out of ordering > would be less (I believe any potential problem > just comes from out of order events). However that may > impact performance. Mentioning that OOO is the main problem here, i.e. that some samples may be misaccounted, should've also been added to the documentation. - Arnaldo