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Newsgroups: comp.lang.ada
Subject: Re: PAR (Was: Embedded languages based on early Ada)
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From: Ray Blaak <rAYblaaK@STRIPCAPStelus.net>
Organization: The Transcend
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Date: Wed, 07 Mar 2007 20:17:40 GMT
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Date: 2007-03-07T20:17:40+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> Why is it bad programming? Consider this:
> 
> declare
>    Sum : Numeric := 0.0;
> begin in parallel
>    Sum := Sum + Integrate (Series (1..N);
>    Sum := Sum + Integrate (Series (N+1..2*N);
>    Sum := Sum + Integrate (Series (2*N+1..3*N);
>    ...
> end;

This is also bad programming. The assignments are in parallel, overwriting
each other, and it is not clear in a given statement what initial value of Sum
is being worked with.

As shown, this example looks like it is after a sequential summation, and that
contradicts what the concurreny execution will do.

A better program would be:

 declare
    Partial1 : Numeric;
    Partial2 : Numeric;
    Partial3 : Numeric;
    Sum : Numeric;
 begin
   begin in parallel
     Partial1 := Integrate (Series (1..N);
     Partial2 := Integrate (Series (N+1..2*N);
     Partial2 := Integrate (Series (2*N+1..3*N);
   end;
   Sum := Partial1 + Partial2 + Partial3;
   ...
 end;

> > Ada already has synchronization constructs that need to be used to allow
> > dependent concurrent items to coordinate properly.
> 
> If so, then what would be the contribution of PAR? 

A syntactic convenience to tediously writing task bodies explicitly. Consider
the immediate example above written out explicitly.

> One important
> proposition is that there is no any use in PAR running absolutely
> independent code. So the question arise, what *exactly* PAR does with:
> 
> 1. ":=" x 2 times
> 2. "+"
> 3. "-"
> 4. "I" x 4 times

The short answer is nothing. PAR spawns some tasks and waits for them to
complete, nothing more or less.

Parallel programing is hard. Relying on implicit optimization is what I
recommend against, since in general it is a fiendishly complex problem as to
how to sort out the interdependencies.

Keep PAR simple, and leave it to the programmer to figure out the
synchronization.

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
rAYblaaK@STRIPCAPStelus.net                    The Rhythm has my soul.