Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756713Ab1DVVED (ORCPT ); Fri, 22 Apr 2011 17:04:03 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:59586 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756647Ab1DVVEA (ORCPT ); Fri, 22 Apr 2011 17:04:00 -0400 Date: Fri, 22 Apr 2011 23:03:35 +0200 From: Ingo Molnar To: Stephane Eranian Cc: Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org, Andi Kleen , Peter Zijlstra , Lin Ming , Arnaldo Carvalho de Melo , Thomas Gleixner , Peter Zijlstra , eranian@gmail.com, Arun Sharma , Linus Torvalds , Andrew Morton Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2 Message-ID: <20110422210335.GB21364@elte.hu> References: <20110422092322.GA1948@elte.hu> <20110422105211.GB1948@elte.hu> <20110422131846.GA20760@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes 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: 3394 Lines: 92 * Stephane Eranian wrote: > Let's go back to your example. > Performance counter stats for './array' (10 runs): > > ?? ? ? ? 6,719,130 cycles:u ? ? ? ? ? ? ? ? ? ( +- ? 0.662% ) > ?? ? ? ? 5,084,792 instructions:u ? ? ? ? ? # ? ? ?0.757 IPC ? ? ( +- ? 0.000% ) > ?? ? ? ? 1,037,032 l1-dcache-loads:u ? ? ? ? ?( +- ? 0.009% ) > ?? ? ? ? 1,003,604 l1-dcache-load-misses:u ? ?( +- ? 0.003% ) > > Looking at this I don't think you can pinpoint which function has a problem > [...] In my previous mail i showed how to pinpoint specific functions. You bring up an interesting question, cost/benefit analysis: > [...] and whether or not there is a real problem. You need to evaluate the > penalty. Once you know that you can estimate any potential gain from fixing > the code. Arun pointed that out rightfully in his answer. How do you know the > penalty if you don't decompose some more? We can measure that even with today's tooling - which doesnt do cost/benefit analysis out of box. In my previous example i showed the cachemisses profile: weight samples pcnt funct DSO ______ _______ _____ _____ ______________________ 1.9 6184 98.8% func2 /home/mingo/opt/array2 0.0 69 1.1% func1 /home/mingo/opt/array2 and here's the cycles profile: samples pcnt funct DSO _______ _____ _____ ______________________ 2555.00 67.4% func2 /home/mingo/opt/array2 1220.00 32.2% func1 /home/mingo/opt/array2 So, given that there was no other big miss sources: $ perf stat -a -e branch-misses:u -e l1-dcache-load-misses:u -e l1-dcache-store-misses:u -e l1-icache-load-misses:u sleep 1 Performance counter stats for 'sleep 1': 70,674 branch-misses:u 347,992,027 l1-dcache-load-misses:u 1,779 l1-dcache-store-misses:u 8,007 l1-icache-load-misses:u 1.000982021 seconds time elapsed I can tell you that by fixing the cache-misses in that function, the code will be roughly 33% faster. So i fixed the bug, and before it 100 iterations of func1+func2 took 300 msecs: $ perf stat -e cpu-clock --repeat 10 ./array2 Performance counter stats for './array2' (10 runs): 298.405074 cpu-clock ( +- 1.823% ) After the fix it took 190 msecs: $ perf stat -e cpu-clock --repeat 10 ./array2 Performance counter stats for './array2' (10 runs): 189.409569 cpu-clock ( +- 0.019% ) 0.190007596 seconds time elapsed ( +- 0.025% ) Which is 63% of the original speed - 37% faster. And no, i first did the calculation, then did the measurement of the optimized code. Now it would be nice to automate such analysis some more within perf - but i think i have established the principle well enough that we can use generic cache events for such measurements. Also, we could certainly add more generic events - a stalled-cycles event would certainly be useful for example, to collect all (or at least most) 'harmful delays' the execution flow can experience. Want to take a stab at that patch? Thanks, 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/