linux-kernel - Re: [PATCH 2/3] perf tools: Spare double comparison of callchain first entry

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Date:	Thu, 16 Jan 2014 10:17:53 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/3] perf tools: Spare double comparison of callchain first entry

Hi Frederic,

On Wed, 15 Jan 2014 17:59:30 +0100, Frederic Weisbecker wrote:
> On Wed, Jan 15, 2014 at 03:23:46PM +0900, Namhyung Kim wrote:
>> On Tue, 14 Jan 2014 16:37:15 +0100, Frederic Weisbecker wrote:
>> > When a new callchain child branch matches an existing one in the rbtree,
>> > the comparison of its first entry is performed twice:
>> >
>> > 1) From append_chain_children() on branch lookup
>> >
>> > 2) If 1) reports a match, append_chain() then compares all entries of
>> > the new branch against the matching node in the rbtree, and this
>> > comparison includes the first entry of the new branch again.
>> 
>> Right.
>> 
>> >
>> > Lets shortcut this by performing the whole comparison only from
>> > append_chain() which then returns the result of the comparison between
>> > the first entry of the new branch and the iterating node in the rbtree.
>> > If the first entry matches, the lookup on the current level of siblings
>> > stops and propagates to the children of the matching nodes.
>> 
>> Hmm..  it looks like that I thought directly calling append_chain() has
>> some overhead - but it's not.
>
> No that's a right concern. I worried as well because I wasn't sure if there
> is more match than unmatch on the first entry. I'd tend to think that the first
> entry endures unmatches most often, in which case calling match_chain() first
> may be more efficient as a fast path (ie: calling append_chain() involves
> one more function call and a few other details).
>
> But eventually measurement hasn't shown significant difference before and
> after the patch.

I think if the sort key doesn't contain "symbol", unmatch case would be
increased as more various callchains would go into a same entry.

>
>> 
>> >
>> > This results in less comparisons performed by the CPU.
>> 
>> Do you have any numbers?  I suspect it'd not be a big change, but just
>> curious.
>
> So I compared before/after the patchset (which include the cursor restore removal)
> with:
>
> 	1) Some big hackbench-like load that generates > 200 MB perf.data
>
> 	perf record -g -- perf bench sched messaging -l $SOME_BIG_NUMBER
>
> 	2) Compare before/after with the following reports:
>
> 	perf stat perf report --stdio > /dev/null
> 	perf stat perf report --stdio -s sym > /dev/null
> 	perf stat perf report --stdio -G > /dev/null
> 	perf stat perf report --stdio -g fractal,0.5,caller,address > /dev/null 
>
> And most of the time I had < 0.01% difference on time completion in favour of the patchset
> (which may be due to the removed cursor restore patch eventually).
>
> So, all in one, there was no real interesting difference. If you want the true results I can definetly relaunch the tests.

So as an extreme case, could you please also test "-s cpu" case and
share the numbers?

Thanks,
Namhyung
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