Re: Concurrency in Gnat 3.05?

[mfeldman@seas.gwu.edu (Michael Feldman)]

In article <dewar.837740961@schonberg>, Robert Dewar <dewar@cs.nyu.edu> wrote:

>That's quite wrong. Serious use of tasking for real-time programs
>generally prefers to completely avoid time slicing. That is why the
>requirement in the real time annex of Ada is that time slicing must
>NOT occur using the default scheduling policy (FIFO_Within_Priorities).

This is interesting, because once again we are left to read between the
lines of the RM. Several Ada 83 implementations - Ada/Ed and
Meridian, to cite two such - explicited provided for time-slicing at
the user's option. 

>Generally time slicing can be made available on many operating systems,
>but it is a bit tricky in DOS, because of limitations in the operating
>system. It certainly would be possible, but since it is not required (in
>fact it is only permitted as an option), it is not a high priority item.

I still find it interesting that this is never stated in the RM, even
as a possibility.  Yes, optional (non-default) dispatching policies
are provided for. And of course, the entire dispatching discussion
has been moved from the Ada 83 Chap. 9 to Annex D.

>If what you want is effective concurrency between multiple threads, there
>are many ways to achieve this in correctly programmed Ada. An Ada program
>that depends on time slicing is non-portable.

I agree, Robert. 

My only quibble is with the need to read about time-slicing in the RM
only between the lines. You said in an earlier post that you're not even
sure whether the threads implementations of GNAT are in conformance
with Annex D, except for the dodge that this "simulates multi-processors."
IMHO, after all the work that went into such a recent RM, one should
not have to guess at these things.:-)

Mike Feldman