linux-kernel - [BUG] perf report: different reports when run on terminal as opposed to script

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Date:	Mon, 29 Oct 2012 12:01:51 -0400
From:	Dhaval Giani <dhaval.giani@...il.com>
To:	Arnaldo de Melo <acme@...radead.org>, mingo@...nel.org,
	Peter Zijlstra <peterz@...radead.org>, namhyung@...nel.org
Cc:	akshay kumar <iit.akshay@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [BUG] perf report: different reports when run on terminal as opposed
 to script

Hi,

As part of a class assignment I have to collect some performance
statistics. In order to do so I run

perf record -g <the program I have to profile>

And in another window, I start 200 threads of the load generator
(which is not recorded by perf)

This generates me statistics that I expect to see, and I am happy. As
this is academia and a class assignment, I need to collect information
and analyze it across different setups. Which of course meant I script
this whole thing, which basically is

for i in all possibilities
do
perf record -g <the program I have to profile> &
WAITPID=$!
for j in NR_THREADS
do
<start load generator> &
KILLPID=$!
done
wait $PID
kill $KILLPID
mv perf.data results/perf.data.$i
done

(This is basic pseudo script of what I am doing), which results me
having my profile being topped by _vscanf() and the function which I
was seeing dominating in the older report dropping down to something
like 5% (as opposed to 16-17%)

Have I misunderstood how perf works? Something deeper? I am currently
on 3.6.3. I can update to the latest upstream and report back. Any
debug code is very welcome. I can also make my toy program and the
scripts available for you to try out.

Thanks!
Dhaval
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