linux-kernel - Re: [patch 0/3] [Announcement] Performance Counters for Linux

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Date:	Fri, 5 Dec 2008 13:13:08 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	a.p.zijlstra@...llo.nl, paulus@...ba.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	akpm@...ux-foundation.org, eranian@...glemail.com,
	dada1@...mosbay.com, robert.richter@....com, arjan@...radead.org,
	hpa@...or.com, rostedt@...dmis.org
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux


* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Fri, 5 Dec 2008 09:42:33 +0100
> 
> > Please let me repeat: it's a _fundamental_ thesis of performance 
> > instrumentation to not disturb the monitored context. Your insistence 
> > on _stopping_ the monitored task breaks that fundamental axiom!
> 
> This is only a problem if you make your measurement quantums too small.

But if you make the measurement long enough - say we make it 100,000 
usecs, then 0.2 usecs of delay between two read()s is insignificant 
statistically, right? It's a 1:500,000 ratio.

Scheduling out a task and back is far more drastic of an effect than any 
new events in 0.2 usecs.

	Ingo
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