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Date: Thu, 25 Sep 2008 16:04:09 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.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>,
       Andrew Morton <akpm@linux-foundation.org>, prasad@linux.vnet.ibm.com,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       "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
References: <33307c790809241403w236f2242y18ba44982d962287@mail.gmail.com> <1222339303.16700.197.camel@lappy.programming.kicks-ass.net> <8f3aa8d60809250733q70561e6agfa3b00da83773e9f@mail.gmail.com> <1222354409.16700.215.camel@lappy.programming.kicks-ass.net> <alpine.LFD.1.10.0809250758400.3265@nehalem.linux-foundation.org> <33307c790809250825u567d3680w682899c111e10ed6@mail.gmail.com> <20080925153635.GA12840@elte.hu> <alpine.LFD.1.10.0809250924460.3265@nehalem.linux-foundation.org> <48DBFC7D.4050208@goop.org> <alpine.LFD.1.10.0809251448340.3265@nehalem.linux-foundation.org> <20080925222548.GA28309@elte.hu> <alpine.DEB.1.10.0809251840430.32742@gandalf.stny.rr.com>
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Steven Rostedt wrote:
> 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? 

Sure.  NTP keeps machines within 1ms (or better) of each other even
though the network latency is much higher and jittery.

    J