linux-kernel - Re: [patch] Performance Counters for Linux, v4

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Message-ID: <18758.53072.197695.277198@cargo.ozlabs.ibm.com>
Date:	Tue, 16 Dec 2008 08:42:40 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Vince Weaver <vince@...ter.net>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>,
	perfctr-devel@...ts.sourceforge.net
Subject: Re: [patch] Performance Counters for Linux, v4

Vince Weaver writes:

> I see a large (2300 instruction) fixed overhead when measuring
> retired instruction count using the "timec" command
> compared to the "pfmon" tool that comes with perfmon3
> (the pfmon tool has essentially no overhead when
> doing aggragate counts).

Looks like timec will be counting the fork() and execvp() system calls
that are used to run your executable, as well as the executable
itself.  The fork() overhead could be removed fairly easily I think,
the execvp would be hard to get rid of without using ptrace() - and
the use of ptrace was one of the things that Ingo et al. objected to
in perfmon3.

Paul.
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