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

[John Woods]

In article <649 at necis.UUCP>, adamm at necis.UUCP (Adam Moskowitz) writes:
> I'm sure this has been asked before, but here goes again anyway . . .
> A friend of mine has asked to help him locate a "real-time UN*X or UN*X-like
> operating system".  I told him that VenturCom "claims" to sell such a beast,
> but some discussion of their product ensued that led to the following
> question:
>     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.
> Is there anything else we're missing?

The real hard-core Real-Time people generally consider the answer to be
"guaranteed response time".  If an event happens, a real-time programmer needs
to know the absolute maximum time that it will take for a response.  If a
system holds interrupts masked while it paints itself green, taking
arbitrarily long, that's no good at all.  A guarantee of "1 second from level
6 interrupt to response" is the right thing, just not very good.  "10 ms from
level 6 interrupt to response" is much nicer.  However, some people do work
that they consider real time, yet they don't require that kind of guarantee
(as long as it isn't a case of sometimes losing the system for an hour or two
until the paint dries :-); for these people, flexible scheduling is
sufficient.

>So, given that, who makes what he needs?  We know that Charles River Data
>Systems has something called UNOS that supposedly does that.  Does it?  Does
>anyone else know of anything else that is available?
> 
UNOS provides two things of interest to people doing real-time (or at least
fast response time) work:  (1) a flexible priority scheduler, which allows you
to set your control process to high priority so that it is the first thing to
run when your interesting event happens, and (2) the ability to install extra
processors in the system, each of which can run a real Real-Time Executive,
because UNOS itself isn't yet deterministic enough (and because, as a
multi-user operating system, it has a lot of overhead that really
time-sensitive applications don't want).  We have a development package for
our "Dedicated Auxilliary Processors" (what most people would call slave
processors) which is oriented to (but not really dependant on) VRTX (Hunter
and Ready's real-time executive).  If you're interested in UNOS itself,
our address is Charles River Data Systems, 983 Concord Avenue,
Framingham MA 01701.  Actually that is our address even if you aren't
interested, too :-).
-- 
John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101
...!decvax!frog!john, john at frog.UUCP, ...!mit-eddie!jfw, jfw at eddie.mit.edu

	Guns don't kill people; I kill people.