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

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Message-Id: <20081205.000716.40104924.davem@davemloft.net>
Date:	Fri, 05 Dec 2008 00:07:16 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	a.p.zijlstra@...llo.nl
Cc:	paulus@...ba.org, mingo@...e.hu, 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: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date: Fri, 05 Dec 2008 09:03:36 +0100

> On Fri, 2008-12-05 at 18:57 +1100, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > So, while most people would not consider two consecutive read() ops to
> > > be close or near the same time, due to preemption and such, that is
> > > taken away by the fact that the counters are task local time based - so
> > > preemption doesn't affect thing. Right?
> > 
> > I'm sorry, I don't follow the argument here.  What do you mean by
> > "task local time based"?
> 
> time only flows when the task is running.

These things aren't measuring time, or even just cycles, they
are measuring things like L2 cache misses, cpu cycles, and
other similar kinds of events.

So these counters are going to measure all of the damn crap
assosciated with doing the read() call as well as the real work
the task does.

That's not useful to people.
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