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From: stein@dhw68k.cts.com (Rick 'Transputer' Stein)
Newsgroups: comp.realtime
Subject: Re: def. real time
Message-ID: <27346@dhw68k.cts.com>
Date: 28 Oct 89 15:02:54 GMT
References: <5417@eos.UUCP> <8214@rosevax.Rosemount.COM>
Reply-To: stein@dhw68k.cts.com (Rick 'Transputer' Stein)
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Organization: Wolfskill & Dowling residence; Anaheim, CA (USA)
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In article <8214@rosevax.Rosemount.COM> bill@bert.Rosemount.COM (William M. Hawkins) writes:
>But seriously, real time computing can also mean that a set of
>distributed (networked) processors all have the same definition
>of time of day, within some uncertainty tolerance.  What is that
>tolerance on a network?  Can it be reduced by using half the time
>for a request - response transaction as the "lead" time for a
>time of day message?
This idea sounds like something I term "Local Area Multicomputer Time (LAMT)."
LAMT is constructed by synchronizing the clock phases on each node in
the multicomputer with a standard reference (A Cesium clock if need be).
Suppose you've got a multicomputer systems built up out of my favorite
iron: Inmos Txxx.  One of those nodes must be hosted by a workstation
of some other server.  So, take the time returned from a "gettimeofday"
system call, and broadcast that value to all nodes in the network, adding
an offset value to the timestamp proportional the to route time between
hops.  When the time is loaded, you've got a local area mulitcomputer
with each node keeping the identical time as all the rest.  Simple no???

The tolerance for LAMT is dependent on your simulation requirements.
Meaning that if you have processes requiring time keeping to 10
usecs, then you better make damn sure that your clocks in each node
count by shorter intervals, and that they don't drift.
>bill@bert.rosemount.com
-- 
Richard M. Stein (aka, Rick 'Transputer' Stein)
Sole proprietor of Rick's Software Toxic Waste Dump and Kitty Litter Co.
"You build 'em, we bury 'em." uucp: ...{spsd, zardoz, felix}!dhw68k!stein