Re: Notions of Time in Computer Architecture

Date	2011-07-02 10:33 +0200
From	Terje Mathisen <"terje.mathisen at tmsw.no">
Newsgroups	comp.arch
Subject	Re: Notions of Time in Computer Architecture
References	<4E0BA344.6000003@SPAM.comp-arch.net> <6s5ud8-ikv2.ln1@ntp6.tmsw.no> <4e0e2a2f.820481@news.aioe.org>
Message-ID	<ko13e8-bm4.ln1@ntp6.tmsw.no> (permalink)

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John Savard wrote:
> 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, please re-read what I wrote:

Having a single clock source is of course doable, what I claimed was 
that with a totally distributed setup, i.e. _no_ central distribution 
point, you can't do the same.

OTOH, even with a central clock there's no way to guarantee that you can 
determine the absolute order of distributed events, you must also have a 
central time stamp authority, and that would be an impossible bottleneck.

You always end up with the need for some other way (i.e. something like 
a cpu id) to arbitrate between "close enough" events, things that happen 
within the clock resolution steps.

Add in relativity and you realize that it might not be even 
theoretically possible to determine which of two events happened "first".

Terje
-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

Thread

Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-06-29 15:12 -0700
  Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-06-29 22:08 -0400
  Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-06-30 14:13 +0200
    Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-06-30 18:19 +0200
    Re: Notions of Time in Computer Architecture ChrisQ <meru@devnull.com> - 2011-06-30 21:23 +0100
      Re: Notions of Time in Computer Architecture Rick Jones <rick.jones2@hp.com> - 2011-06-30 19:42 +0000
        Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-01 10:46 +0200
          Re: Notions of Time in Computer Architecture Rick Jones <rick.jones2@hp.com> - 2011-07-01 15:17 +0000
            Re: Notions of Time in Computer Architecture ChrisQ <meru@devnull.com> - 2011-07-04 16:51 +0100
      Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-01 05:54 +0100
    Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-06-30 22:42 -0700
    Re: Notions of Time in Computer Architecture jsavard@excxn.aNOSPAMb.cdn.invalid (John Savard) - 2011-07-01 20:16 +0000
      Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-02 10:33 +0200
        Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-02 09:26 +0100
        Re: Notions of Time in Computer Architecture jsavard@excxn.aNOSPAMb.cdn.invalid (John Savard) - 2011-07-02 12:40 +0000
          Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-02 16:14 +0100
          Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-02 16:48 -0400
            Re: Notions of Time in Computer Architecture Quadibloc <jsavard@ecn.ab.ca> - 2011-07-02 14:27 -0700
              Re: Notions of Time in Computer Architecture Stephen Sprunk <stephen@sprunk.org> - 2011-07-03 15:07 -0500
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-04 08:15 +0200
                Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-07-04 17:50 +0200
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-04 21:21 +0200
                Re: Notions of Time in Computer Architecture Robert Wessel <robertwessel2@yahoo.com> - 2011-07-04 23:44 -0500
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-05 07:51 +0200
                Re: Notions of Time in Computer Architecture Robert Wessel <robertwessel2@yahoo.com> - 2011-07-05 01:56 -0500
            Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-07-03 19:28 +0200
          Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 11:27 -0700
            Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-03 22:13 +0100
              Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 18:53 -0700
                Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-04 08:34 +0100
            Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 18:51 -0700
            Re: Notions of Time in Computer Architecture anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-07-04 09:20 +0000
              Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-04 12:26 -0700
                Re: Notions of Time in Computer Architecture EricP <ThatWouldBeTelling@thevillage.com> - 2011-07-04 19:35 -0400
                Re: Notions of Time in Computer Architecture Bernd Paysan <bernd.paysan@gmx.de> - 2011-07-08 15:48 +0200
            Re: Notions of Time in Computer Architecture Michael S <already5chosen@yahoo.com> - 2011-07-05 15:24 -0700
              Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-08 09:25 -0700
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-08 12:12 -0700
                Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-08 17:27 -0700
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-09 18:07 -0400
                Re: Notions of Time in Computer Architecture EricP <ThatWouldBeTelling@thevillage.com> - 2011-07-10 12:03 -0400
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-10 13:02 -0700
    Re: Notions of Time in Computer Architecture Bernd Paysan <bernd.paysan@gmx.de> - 2011-07-08 15:13 +0200
  Re: Notions of Time in Computer Architecture Gubbi <gubsan100@hotmail.com> - 2011-07-01 00:50 -0700

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