linux-kernel - Re: [PATCH 0/4] perf tools: New comm infrastructure

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Date:	Thu, 12 Sep 2013 22:36:58 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
	David Ahern <dsahern@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/4] perf tools: New comm infrastructure


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> The way we handle hists sorted by comm is to first gather them by tid 
> then in the end merge/collapse hists that end up with the same comm.
> 
> But merging hists has shown some performances issues, especially with 
> callchain where the operation can be very heavy.
> 
> So this new comm infrastructure aims at removing comm collapses. It 
> brings two features:
> 
> 1) Keep track of comms lifecycle by storing timestamps when the comms 
> are set. This way we can map the precise comm to any thread:time couple. 
> This only works if the PERF_SAMPLE_ID comes along comm and fork events, 
> otherwise we only track the latest comm set for a thread.
> 
> This can provide us more precise comm sorted hists by distinguishing pre 
> and post exec timeframes into seperate hists for a single thread.
> 
> Note that although the comm infrastructure is ready to do this, I 
> haven't yet made the perf tools support that. It's a TODO entry.
> 
> 2) Allocate comms only once instead of duplicating them for all threads 
> sharing a same one. Two threads having the same comm should now point to 
> the same string. As a result we can compare hists thread comm by 
> address.
> 
> The big upside is that we can now live sort comm hists instead of 
> collapsing them in the end of the processing.
> 
> I've seen very nice performance results on perf report. Roughly a 1.5x 
> to 2x on perf report default stdio output with callchains.
> 
> You can try this branch:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 	perf/comm
> 
> May be merging that with Namhyung callchains patches could provide some
> cumulative nice results.

It would be nice to try Linus's testcase, which is, in essence a kernel 
build profile:

    make defconfig
    perf record -g make -j64 bzImage

and to make sure that it can analyze the data in same, non-annoying 
runtimes. What I saw was 30 minutes of runtime - a 2x improvement is not 
nearly enough, 15 minutes is still an eternity.

Thanks,

	Ingo
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