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

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Date:	Fri, 7 Aug 2009 13:29:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
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


* 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)

	Ingo
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