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

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Message-ID: <20090629235500.GB5869@elte.hu>
Date:	Tue, 30 Jun 2009 01:55:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Vince Weaver <vince@...ter.net>
Cc:	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> wrote:

>> Besides, you compare perfcounters to perfmon
>
> what else shoud I be comparing it to?
>
>> (which you seem to be a contributor of)
>
> is that not allowed?

Here's the full, uncropped sentence i wrote:

 " Besides, you compare perfcounters to perfmon (which you seem to 
   be a contributor of), while in reality perfmon has much, much 
   worse (and unfixable, because designed-in) measurement overhead. "

Where i question the blatant hypocracy of bringing up perfmon as a 
good example while in reality perfmon has far worse measurement 
overhead than perfcounters, for a wide range of workloads.

As far as i can see you didnt answer my questions: why are you 
dismissing perfcounters for a minor, once per startup measurement 
offset (which is entirely fixable - see the patch i sent), while you 
generously allow perfmon to have serious, 90% measurement overhead 
amounting to billions of instructions overhead per second, for 
certain workloads?

	Ingo
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