Date: Fri, 05 Dec 2008 00:49:22 -0800 (PST)
Message-Id: <20081205.004922.244597525.davem@davemloft.net>
To: mingo@elte.hu
Cc: a.p.zijlstra@chello.nl, paulus@samba.org, tglx@linutronix.de,
       linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
       akpm@linux-foundation.org, eranian@googlemail.com, dada1@cosmosbay.com,
       robert.richter@amd.com, arjan@infradead.org, hpa@zytor.com,
       rostedt@goodmis.org
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux
From: David Miller <davem@davemloft.net>
In-Reply-To: <20081205084233.GE2030@elte.hu>
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	<20081205084233.GE2030@elte.hu>
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From: Ingo Molnar <mingo@elte.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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