Real-time UNIX - what is it & who has it?

[Ian Kluft]

/ adamm at necis.UUCP (Adam Moskowitz) / writes:
> A friend of mine has asked to help him locate a "real-time UN*X or UN*X-like
> operating system".
 [ ... ]
>     Assuming that you are doing something like data acquisition or
>     process control, what is required to make an O/S "real-time"?
>
> The answer he and I came up with was this: the ability to have absolute
> control over the scheduling of processes.

Two issues to note when looking for a real-time OS are
   kernel pre-emption
   context switching time

Kernel pre-emption is an important one to consider on Unix because most
Unix systems do not allow system calls to be interrupted.  Of course,
context-switching time is somewhat obvious because it affects the time
between arrival of an interrupt and (re)starting a user process to handle
the event.

That's really what it comes down to.  If a guarantee can be made of the
amount of time it takes between an event and entering the code to handle
it, an OS is considered real-time.  Of course, if that time is too long,
it isn't worth mentioning.

[End of unbiased material]

While everyone was mentioning the real-time Unix's they knew of, I thought
I'd mention the one I work with.  Hewlett-Packard's HP-UX is a full System
V with extentions for BSD 4.2 and real-time.  While I don't have the exact
numbers for measured real-time response, I remember that they are measured
in milliseconds, as would be expected.

------------------------------------------------------------
    Ian Kluft			RAS Lab
    hplabs!hprasor!kluft	HP Network Systems Group
    kluft at hpda.hp.com		Cupertino, CA
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