linux-kernel - Re: performance counter ~0.4% error finding retired instruction count

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Message-ID: <20090627172854.GE21595@elte.hu>
Date:	Sat, 27 Jun 2009 19:28:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Vince Weaver <vince@...ter.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: performance counter ~0.4% error finding retired instruction
	count


* Paul Mackerras <paulus@...ba.org> wrote:

> Ingo Molnar writes:
> 
> > I measured 2000, but generally a few thousand cycles per 
> > invocation sounds about right.
> 
> We could actually do a bit better than we do, fairly easily.  We 
> could attach the counters to the child after the fork instead of 
> the parent before the fork, using a couple of pipes for 
> synchronization.  And there's probably a way to get the dynamic 
> linker to resolve the execvp call early in the child so we avoid 
> that overhead.  I think we should be able to get the overhead down 
> to tens of userspace instructions without doing anything 
> unnatural.

Definitely so.

	Ingo
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