Real time and concurrency (was: What is real-time?)

[pipex!warwick!uknet!yorkohm!minster!mjl-b@uunet.uu.net]

In article <1992Oct22.153813.15179@mksol.dseg.ti.com> mccall@mksol.dseg.ti.com 
(fred j mccall 575-3539) writes:
>In <719335132.5132@minster.york.ac.uk> mjl-b@minster.york.ac.uk writes:
>>The Burns and Wellings book writes off C fairly early on, because it has no
>>features for concurrency. Real-time and concurrency go hand in hand, and the
>>authors provide a fairly convincing argument.
>
>How many languages (other than Ada) actually provide support for
>concurrency (as opposed to getting it from the OS)?

The book covers Ada, Modula, Modula-2 and occam, all of which provide
concurrency features. Other concurrent languages get a mention, but the
above are the "main" langauges used.

>Does this mean
>that there was no such thing as real time software until Ada was
>available? 

Of course not. My point is that a concurrent language makes the expression
and maintenance of real-time programs far easier. Real time software
is most often inherently concurrent, and it makes the job so much harder if
those concurrent tasks have to be forced into a sequential form.

Not only have you lost the inherent structure of the design in the
implementation, the expression of other real-time concerns such as deadline
constraints on a particular task becomes much harder.

For example, you have three tasks that must run concurrently: P, Q and R. If
you have a concurrent language, you simply map them to the unit of
concurrency. In a sequential language, you must split up P, Q and R into,
say, three bits, so that you can interleave their execution:

P1;
Q1;
R1;
P2;
Q2;
R2;
P3;
Q3;
R3;

Now try and express the constraint that P must complete its execution within
2.0 seconds of when it started... not impossible, but already the program's
difficult to understand and maintain.

The trade off is between manually interleaving P, Q and R at the coding
stage, and letting the language do it at run time. I know which one I'd
rather maintain :-)

>An awful lot of real-time software has been written in
>LANGUAGES to provide no support for concurrency (like assembly).

Yes, it has. But how easy was it to write, debug and maintain?

>There seems to be something a bit off about this line of reasoning.

The reasoning is better explained in Burns and Wellings' book -- read it and
then make your judgement.

>Fred.McCall@dseg.ti.com - I don't speak for others and they don't speak for me
.

Mat

| Mathew Lodge                      | "A conversation with you, Baldrick,     |
| mjl-b@minster.york.ac.uk          |  and somehow death loses its sting..."  |
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