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

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Date:	Fri, 05 Dec 2008 00:49:22 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	mingo@...e.hu
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

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.

Furthermore, there are multiple registers and states to update
atomically when a perf counter overflows.  You're read/write thing
just doesn't cut it, especially for certain kinds of hardware.

It's really a utopian view of the world. :)

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