Re: Ada Annex E (Just curious :-)

[Jeffrey Carter <jrcarter@acm.org>]

Robert A Duff wrote:
> 
> Jeffrey Carter <jrcarter@acm.org> writes:
> 
> > One can argue that Annex E is completely superfluous. One could have a
> > compiler that targeted a distributed system, and converted all intertask
> > communications (rendezvous and protected operations) into network
> > messages. A program for a distributed system could look exactly like a
> > program for a multiprocessor, which would make porting from one to the
> > other much simpler.
> 
> One can argue that, but I don't think it's true.  ;-)
> I implemented such a beast for Ada 83.  You could put tasks
> on different nodes of a distributed system.  You could control where
> they ran, or let the system choose (some sort of primitive load
> balancing).  Rendezvous was implemented in terms of message passing
> across the network.
> 
> But we had to "cheat" a little bit.  Above, you mention "rendezvous and
> protected operations" as the intertask communications mechanisms.  But
> there's another one: shared variables.  In Ada (83 and 95) it is
> perfectly legitimate for two tasks to refer to the same variable (so
> long as they synchronize properly, perhaps using rendezvous).  But the
> compiler can't tell which variables are shared.  This pretty much
> implies that all tasks must share the same address space.  (You *could*
> implement distributed shared memory in software, but it would be
> intolerably slow.  And you would have to pay that price even for
> variables local to a single task (or node), because the compiler can't
> tell which variables are shared).

That's true, but I think disallowing shared variables (except protected
objects) is reasonable for all systems :)

-- 
Jeff Carter
"You empty-headed animal-food-trough wiper."
Monty Python & the Holy Grail