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From: stein@dhw68k.cts.com (Rick 'Transputer' Stein)
Newsgroups: comp.realtime
Subject: Re: Non-determinism not "bad", per se. (Was: What is "real-time" really?)
Message-ID: <30672@dhw68k.cts.com>
Date: 15 Mar 90 03:31:15 GMT
References: <98692@linus.UUCP> <1990Feb24.195542.21454@newcastle.ac.uk> <98987@linus.UUCP> <8178@pt.cs.cmu.edu> <53078@sgi.sgi.com>
Reply-To: stein@dhw68k.cts.com (Rick 'Transputer' Stein)
Organization: Wolfskill & Dowling residence; Anaheim, CA (USA)
Lines: 40

In article <53078@sgi.sgi.com> rpw3@sgi.com (Rob Warnock) writes:
>Non-deterministic means that you cannot (even theoretically) predict
>the exact sequence of system states; it does *not* mean that you cannot
>predict the behavior of output variables within a useful epsilon (or in
>some cases even exactly) or that the output variables will necessarily
>be wrong.
I generally agree.  I once wrote a multicomputer simulation where the message
passing was hosed-up.  The sim started to converge, but all of a sudden, it
bounced around, started to converge, bounced around, etc...  And this was
just a simple numerical solution to a PDE with real clean boundary conditions.

It was like a highly nonlinear (e.g., chaotic) system.  You know, strange
attractors and the like.  That's what I call non-deterministic: failure
of a simulation to provide the identical answers for a fixed set of input
conditions each time you run it.  That's what you get when your message-passing
is hosedup.
>
>If you have parallel processors observing the same external inputs, you must
>anticipate and provide for the fact that they will *not* always detect external
>transitions at the same system clock time. If these inputs cause interrupts,
Ah! My favorite subject, parallel systems with clocks.  Can you imagine
running a real-time multicomputer simulation.  I'm talking about 64K separate
processors, each with its own unique clock.  How do you synchronize all
these cpus to within, say 64 usecs?  Kinda' like lining up 64K people with
different watches and asking them what time is it?  You'll need "split-second"
synchronization for real-time multicomputers.

But I think Warnock is talking about the shared-memory ghettoblaster variety,
and these sucker all derive strobes from a single source (chicken-shit if
you ask me :-}).

>Of course, nobody said designing such systems to be correct was easy...  ;-}
Its a bitch, trust me!

>-Rob
--rick
-- 
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