Re: GNAT R/T Annex and Win95

[labtek@cs.yale.edu (Tom Griest)]

"Marin David Condic, 407.796.8997, M/S 731-93" <condicma@PWFL.COM> writes:

[snip]

>    Do you consider Windows NT capable of being a "realtime" operating
>    system? (It doesn't seem to be advertised as such.... yet.)

That's correct.  NT is not designed for real-time use.  But many are
using it for soft real-time applications.

>    We'd like to build some of our data systems and ground support
>    systems around Ada and NT, but we're not sure we can get the
>    needed behavior out of it for a realtime system. (GNAT might even
>    be usable here since most of this is "in house" development and
>    not a contract deliverable. More leeway with what you do in this
>    case.)
>
>    We need to know things like "If I execute a delay statement, will
>    I wake up and have a deterministic span of time between the clock
>    going off and my code resuming?" Similar questions for interrupt
>    processing and such. Or "will the OS dynamically rescale my task
>    priorities and screw everything up?"

No.  NT does not do this.  Of course this means you can lock out rather
important operations... but usually your real-high priority stuff should
be fairly short duration.

> Or priority inversions that
>    can occur if a low priority thread uses an uninterruptable OS
>    routine. Lots of things are uncertain about NT WRT its use as a
>    realtime OS.

True.  There are multithreaded libraries, but you always have to
be aware of any locks that might be held.  NT does not have priority
inheritence.

>    (Of course, you can always buy a processor that executes a
>    quintillion instructions per second and hope the latencies never
>    get big enough to matter! ;-)

Or you can take a two-processor system and run NT on one and some
real-time code on the other, using shared memory and interprocessor
interrupts.

>    Opinions? I'd really like to have some best guesses to pass on to
>    the folks here who do these sort of systems.

In the end it depends on two things:
  1) what happens if you miss a deadline
  2) how much laxity do you have in your timeline?

If the answer to #1 is: loss of life/property, I'd recommend not using NT.
On the other hand, if it is system runs a little slower then you're probably
ok depending on #2.

-Tom