linux-kernel - Re: performance counter 20% error finding retired instruction count

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Date:	Fri, 26 Jun 2009 14:22:40 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: performance counter 20% error finding retired instruction count

On Wed, 24 Jun 2009, Ingo Molnar wrote:
> * Vince Weaver <vince@...ter.net> wrote:
>
> Those ~2100 instructions are executed by your app: as the ELF
> dynamic loader starts up your test-app.
>
> If you have some tool that reports less than that then that tool is
> not being truthful about the true overhead of your application.

Wait a second... my application is a statically linked binary.  There is 
no ELF dynamic loader involved at all.

On further investigation, all of the overhead comes _entirely_ from the 
perf utility.  This is overhead and instructions that would not occur when 
not using the perf utility.

>From the best I can tell digging through the perf sources, the performance 
counters are set up and started in userspace, but instead of doing an 
immediate clone/exec, thousands of instructions worth of other stuff is 
done by perf in between.

Ther "perfmon" util, plus linux-user simulators like qemu and valgrind do 
things properly.  perf can't it seems, and it seems to be a limitation of 
the new performance counter infrastructure.


Vince

PS.  Why is the perf code littered with many many  __MINGW32__ defined?
      Should this be in the kernel tree?  It makes the code really hard
      to follow.  Are there plans to port perf to windows?


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