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

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Message-ID: <19013.49344.860564.948905@cargo.ozlabs.ibm.com>
Date:	Sat, 27 Jun 2009 16:48:32 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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.

Paul.
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