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Date: Thu, 25 Sep 2008 18:45:53 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Ingo Molnar <mingo@elte.hu>
cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Jeremy Fitzhardinge <jeremy@goop.org>, Martin Bligh <mbligh@google.com>,
       Peter Zijlstra <peterz@infradead.org>, Martin Bligh <mbligh@mbligh.org>,
       linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
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       "Frank Ch. Eigler" <fche@redhat.com>, David Wilder <dwilder@us.ibm.com>,
       hch@lst.de, Tom Zanussi <zanussi@comcast.net>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer
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On Fri, 26 Sep 2008, Ingo Molnar wrote:
> 
> Firstly they need a low-frequency (10khz-100khz) shared clock line 
> across all CPUs. A single line - and since it's low frequency it could 
> be overlaid on some existing data line and filtered out. That works 
> across NUMA nodes as well and physics allows it to be nanosec accurate 
> up to dozens of meters or so.

Can this possibly be true? I mean, light travels only one foot every
nanosecond. Can it really keep nanosecond accuracy up to dozens of  meters 
away? If you send the same signal to CPU1 that is 1 foot away, as well as 
send it to CPU2 that is 2 feet away. CPU2 will get that signal at least 1 
nanosec after CPU1 receives it.

Of course if the hardware is smart enough to know this topology, then it 
could account for these delays in traffic.

-- Steve