linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Thu, 25 Sep 2008 18:45:53 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer


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

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