Over in comp.arch, this was mentioned as how one might get time to the far reaches of a chip with equal delay. I recall that someone not too long ago wanted to distribute a PPS signal to a bunch of systems, and thought this might be helpful.
rick jones [ This is a repost of the following article: ] [ From: [email protected] (John Savard) ] [ Subject: Re: Notions of Time in Computer Architecture ] [ Newsgroups: comp.arch ] [ Message-ID: <[email protected]> ] On Thu, 30 Jun 2011 14:13:23 +0200, Terje Mathisen <"terje.mathisen at tmsw.no"> wrote, in part: >With the sub-ns time scales in modern cpus, any large cluster (even if >presented as a single image CC-NUMA system to the programmer) will get >into trouble here: > >You cannot have a globally consistent, totally distributed clock: Unless >you designate a single point of control, there's no way to always be >able to figure out the absolute order of two or more distributed events, >right? Actually, no. This is not really a problem. If you want to have a clock distributed on a chip that's accurate to a small fraction of the time that light takes to travel across the chip, you *can* do it. Think of the letter H. Assume you have a pin on the edge of the chip where an external high-frequency clock signal comes in. Take that to the exact center of traces having the form of the letter H. Clearly, when the signal reaches the four corners of the H, it will have travelled an equal distance in each of the four cases. Now, make each of those four corners the center of a smaller letter H. Repeat (but not infinitely, as we don't need a _true_ fractal.) In fact, this actually *is* how clock signals are distributed on some chips. John Savard http://www.quadibloc.com/index.html -- oxymoron n, commuter in a gas-guzzling luxury SUV with an American flag these opinions are mine, all mine; HP might not want them anyway... :) feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH... _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
