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From: jburnes@crash.cts.com (Jim Burnes)
Newsgroups: comp.realtime
Subject: Re: My IBM rep says that the RS/6000/w AIX is realtime, is this true?
Message-ID: <4192@crash.cts.com>
Date: 30 Aug 90 23:57:14 GMT
References: <4350@cica.cica.indiana.edu> <1793@abvax.UUCP> <467@rafos.UUCP>
Organization: Crash TimeSharing, El Cajon, CA
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Skip:

About your question about the RS/6000 and AIX, I really don't think the
thing has what most sentient engineers mean when they say real-time.
As far as I know AIX is really a descendent of the sysv series of unix
kernels which are definately not realtime.  Also never take an IBM reps
word for anything.  The real question that picks the realtime machines
from the ones that 'up your statistical priority' is that realtime kernels
have (almost always) preemtive priority multitasking.  In other words,
realtime kernels are deterministic, not stochastic.  Easy example:

    Astronaut Aldrin is aboard a shuttle and is playing chess with the
onboard computer.  The computer is really crunching away trying to 
find the best move to put Aldrin in check when the task scheduler on
the system sees that its time to do a three second orbital burn.
That burn MUST commence in 37 milliseconds.  On your basic unix box
the astronaut would most likely be stranded in space for ever.  With
the miracles of a realtime kernel the chess program having a lower
priority than the orbital manuevering program (hopefully) means that
the chess program gets swapped out NOW every time this situation occurs.
On most flavors of unix, you would have pumped the orbital program up
to the highest priority and chess would be running at normal or low
priority.  When the orbital burn timer tells the system its time to 
swap in the manuevering program the manuevering program will be swapped in
when the unix kernel thinks the chess program has had enough time
allocated to it.  Now by the OS's computations, the chess program 
deserves another 43 milliseconds.  When the orbital manuvering program
finalllxy gets swapped in and running, its too late to do the orbital
burn and the shuttle and Aldrin skip off the atmosphere, doomed to
play out endless series of chess matches with each other. 

Hope I didnt insult your intelligence or mine with this little explanation.

Have a good one.