linux-kernel - RE: [patch] x86, perf_counter, bts: add bts to perf

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Date:	Fri, 7 Aug 2009 13:14:32 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Friday, August 07, 2009 1:30 PM
>To: Metzger, Markus T
>Cc: Peter Zijlstra; tglx@...utronix.de; hpa@...or.com; markus.t.metzger@...il.com; linux-
>kernel@...r.kernel.org
>Subject: Re: [patch] x86, perf_counter, bts: add bts to perf_counter
>
>
>* Ingo Molnar <mingo@...e.hu> wrote:
>
>> btw., the number of samples seems to be varying too widely:
>>
>> titan:~> for ((i=0;i<10;i++)); do perf record -f -e branches:u -c 1 true 2>/dev/null; perf report |
>head -1; done
>> # Samples: 28784
>> # Samples: 24063
>> # Samples: 22788
>> # Samples: 30449
>> # Samples: 15335
>> # Samples: 30557
>> # Samples: 24010
>> # Samples: 23866
>> # Samples: 24877
>> # Samples: 24330
>>
>> compared to the branch-stat itself:
>>
>> titan:~> perf stat -v --repeat 10 -e branches:u true
>> [ perf stat: executing run #1 ... ]
>> [ perf stat: executing run #2 ... ]
>> [ perf stat: executing run #3 ... ]
>> [ perf stat: executing run #4 ... ]
>> [ perf stat: executing run #5 ... ]
>> [ perf stat: executing run #6 ... ]
>> [ perf stat: executing run #7 ... ]
>> [ perf stat: executing run #8 ... ]
>> [ perf stat: executing run #9 ... ]
>> [ perf stat: executing run #10 ... ]
>>
>>  Performance counter stats for 'true' (10 runs):
>>
>>           23851  branches                   ( +-   0.000% )
>>
>>     0.000639653  seconds time elapsed   ( +-   2.474% )
>>
>> do we lose records in the recording?
>
>i doubt it's lost records. Even with SCHED_FIFO sampling and with a
>huge, 512 MB mmap ring-buffer we get a BTS sample count variation in
>the +- 10% range:
>
>titan:/home/mingo> for ((i=0;i<10;i++)); do perf record -r 1 -m
>131072 -f -e branches:u -c 1 true 2>/dev/null; perf report | head
>-1; done
># Samples: 24860
># Samples: 24177
># Samples: 26165
># Samples: 25682
># Samples: 29175
># Samples: 23136
># Samples: 27102
># Samples: 29888
># Samples: 25524
># Samples: 24266
>
>(and these are all just user-mode executions)


Hmmm, we do get a single branch record for the transition from kernel to user mode.
We should therefore expect some deviation, but 10% sounds too much to me.

Perf record traces itself and then exec's the application you want to trace.
So, you would get a mix of perf trace and actual application trace. That could explain
a 10% deviation.

Perf stat, on the other hand, seems to go through some effort to only trace the
actual application.

What do you think?

In my tests, I'm looking for deterministic trace, e.g. on-the-spot branches for
for(;;) {} loops or a branch to 0 for a null function pointer call. I pretty much
ignored the 'noise' around that.


regards,
markus.

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