linux-kernel - Re: [numbers] perfmon/pfmon overhead of 17%-94%

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Date:	Fri, 03 Jul 2009 20:31:04 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Vince Weaver <vince@...ter.net>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>
Subject: Re: [numbers] perfmon/pfmon overhead of 17%-94%

Vince Weaver <vince@...ter.net> writes:
>
> as I said in a previous post, on most x86 chips the instructions_retired
> counter also includes any hardware interrupts that occur during the
> process runtime.

On the other hand afaik near all chips have interrupt performance counter
events.

So if you're willing to waste one of the variable counter registers 
you can always count those and then correct based on the other count.

But the question is of course if it's worth it, the error should 
be really small. Also you could always lose a few cycles occasionally
in other "random" events, which can happen too.

>  So any clock interrupts, etc, show up as an extra
> instruction.  So on the "million" benchmark, it's usually +/- 2 extra
> instructions.

1-2 error in a million doesn't sound like a catastrophic problem.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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