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From: Maciej Sobczak <no.spam@no.spam.com>
Newsgroups: comp.lang.ada
Subject: Re: question about tasks, multithreading and multi-cpu machines
Date: Fri, 17 Mar 2006 09:22:21 +0100
Organization: CERN - European Laboratory for Particle Physics
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Date: 2006-03-17T09:22:21+01:00
List-Id: <comp.lang.ada>

Jeffrey R. Carter wrote:

>> The difference between "task" and "thread" is that "task" is something 
>> to do, whereas "thread" is a way to do it.
> 
> The way to design SW in a decent language such as Ada is to create in SW 
> a model of the problem. If your problem includes multiple tasks to be 
> completed concurrently, then your SW model of the problem should include 
> multiple objects which are models of these tasks and which execute 
> concurrently.

Makes sense, but is biased towards single programming paradigm.

Consider a simple example of two long vectors that need to be added. In 
the simplest case you do this:

    for I in Vector'Range loop
       V3(I) := V1(I) + V2(I);
    end loop;

and you're done.

Now, assume that you want to target dual-CPU machine and you *know* that 
you could greatly benefit from making things in paraller.

Do you see any need to model the above sentence using additional objects?
I don't, because in the *application domain* no additional type nor 
object was created by just computing things in paraller.

That's why the "lightweight concurrency", like in Occam, would be more 
appropriate here. Something like this:

declare
    procedure Add_Range(First, Last : in Index_Range) is
    begin
       for I in First .. Last loop
          V3(I) := V1(I) + V2(I);
       end loop;
    end Add_Range;
begin
    Add_Range(Vector'First, Vector'Last / 2);
with
    Add_Range(Vector'Last / 2 + 1, Vector'Last);
end;

(syntax taken from the top of my head)

I see the above as better expressing what is really done.
The same example using additional types or objects would not make the 
code more readable for me.


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/