UNIX real-time performance

[utzoo!duke!decvax!ucbvax!ron at NOSC-CC]

From: ron at NOSC-CC (Ronald L. Broersma)

	We are doing some real-time UNIX work here.  We have a very large
ocean simulator which is used for advanced torpedo development.  This
simulator responds to real torpedo hardware and therefore must run real
time.  The simulator is distributed among numerous machines but the tail
end of it all is a PDP 11/70 running (almost) vanilla PWB from Bell.
As a bus terminator it has an ADAGE GP400 graphics processor.  Is also
has 3 IEEE-488 bus interfaces to various instrumentation points in the
simulator and torpedo hardware.  The task is to display what is going
on in the (simulated) water in real time and to totally log all the data
on disk so that it can be replayed if necessary and to impress admirals.
The critical response was needed mostly to keep the display from flickering.
The ADAGE interrupts the processor after ever frame (60 hz) and will
flicker if the interrupt is not responded to almost immediately.

	Our conclusions were that UNIX is not a real time operating system.
We actually knew this ahead of time but preferred UNIX for the development.
However, if you know a little about the internals of UNIX and are willing
to perform a few minor hacks you can do some real-time things very nicely.
We did have some numbers on speed for sys calls and context switches as
measured by scope but I can't find them anywhere.  If you're really
interested, I could perform the measurements again.

--Ron
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