Date: Fri, 26 Jun 2009 14:22:40 -0400 (EDT)
From: Vince Weaver <vince@deater.net>
To: Ingo Molnar <mingo@elte.hu>
cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Mackerras <paulus@samba.org>,
       linux-kernel@vger.kernel.org
Subject: Re: performance counter 20% error finding retired instruction count
In-Reply-To: <20090624151010.GA12799@elte.hu>
Message-ID: <Pine.LNX.4.64.0906261417560.23467@pianoman.cluster.toy>
References: <Pine.LNX.4.64.0906240937120.10620@pianoman.cluster.toy>
 <20090624151010.GA12799@elte.hu>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1522
Lines: 38

On Wed, 24 Jun 2009, Ingo Molnar wrote:
> * Vince Weaver <vince@deater.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?


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/