Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754095Ab1DWNQl (ORCPT ); Sat, 23 Apr 2011 09:16:41 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:58185 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753647Ab1DWNQk convert rfc822-to-8bit (ORCPT ); Sat, 23 Apr 2011 09:16:40 -0400 Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2 From: Peter Zijlstra To: Stephane Eranian Cc: Andi Kleen , Ingo Molnar , arun@sharma-home.net, Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org, Lin Ming , Arnaldo Carvalho de Melo , Thomas Gleixner , eranian@gmail.com, Arun Sharma , Linus Torvalds , Andrew Morton In-Reply-To: References: <20110422092322.GA1948@elte.hu> <20110422105211.GB1948@elte.hu> <20110422165007.GA18401@vps.sharma-home.net> <20110422203022.GA20573@elte.hu> <20110422203222.GA21219@elte.hu> <20110423000347.GC9328@tassilo.jf.intel.com> <1303545012.2298.44.camel@twins> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Sat, 23 Apr 2011 15:16:01 +0200 Message-ID: <1303564561.2298.62.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2978 Lines: 63 On Sat, 2011-04-23 at 14:06 +0200, Stephane Eranian wrote: > On Sat, Apr 23, 2011 at 9:50 AM, Peter Zijlstra wrote: > > On Fri, 2011-04-22 at 17:03 -0700, Andi Kleen wrote: > >> > > Yes, and note that with instructions events we even have skid-less PEBS > >> > > profiling so seeing the precise . > >> > - location of instructions is possible. > >> > >> It was better when it was eaten. PEBS does not actually eliminated > >> skid unfortunately. The interrupt still occurs later, so the > >> instruction location is off. > >> > >> PEBS merely gives you more information. > > > > You're so skilled at not actually saying anything useful. Are you > > perchance referring to the fact that the IP reported in the PEBS data is > > exactly _one_ instruction off? Something that is demonstrated to be > > fixable? > > > > Or are you defining skid differently and not telling us your definition? > > > > PEBS is guaranteed to return an IP that is just after AN instruction that > caused the event. However, that instruction is NOT the one at the end > of your period. Let's take an example with INST_RETIRED, period=100000. > Then, the IP you get is NOT after the 100,000th retired instruction. It's an > instruction that is N cycles after that one. There is internal skid due to the > way PEBS is implemented. > > That is what Andi is referring to. The issue causes bias and thus impacts > the quality of the samples. On SNB, there is a new INST_RETIRED:PREC_DIST > event. PREC_DIST=precise distribution. It tries to correct for the skid > on this event on INST_RETIRED with PEBS (look at Vol3b). Sure, but who cares? So your period isn't exactly what you specified, but the effective period will have an average and a fairly small stdev (assuming the initial period is much larger than the relatively few cycles it takes to arm the PEBS assist), therefore you still get a fairly uniform spread. I don't much get the obsession with precision here, its all a statistics game anyway. And while you keep saying the examples are too trivial and Andi keep sprouting vague non-statements, neither of you actually provide anything sensible to the discussion. So stop f*cking whining and start talking sense or stop talking all together. I mean, you were in the room where Intel presented their research on event correlations based on pathological micro-benches. That clearly shows that exact event definitions simply don't matter. Similarly all this precision wanking isn't _that_ important, the big fish clearly stand out, its only when you start shaving off the last few cycles that all that really comes in handy, before that its mostly: ooh thinking is hard, lets go shopping. -- 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/