[MPI3 Fortran] Argument data sizes

[Bill Long]

Jeff Squyres wrote:
>
> Let's also acknowledge that a significant "artificial constraint" is 
> backwards compatibility for existing applications.  We can't just 
> change all the C bindings to make the millions of lines of MPI code 
> out there no longer be compilable.

I agree that backward compatibility is a good thing.  However, are not 
the C bindings insulated from this?  If the new interface specifies 
intptr_t  for the count argument, and the existing program supplies an 
int (conforming to the old standard) the argument will be automatically 
promoted in the caller.  I guess I don't see a problem there.


>
> Let's be fair -- *parallel programming* is hard.  

Yes, certainly true.

> MPI fits some kinds of parallel program models very well; MPI is a bad 
> match for other kinds of parallel program models.  The same can be 
> said for all parallel programming systems.  I think the last 20+ years 
> have shown that there is no one-size-fits-all for parallel programming.

After many years of attrition, the still standing parallel models seem 
to be shared memory (OpenMP, which works for small numbers of 
processors) and SPMD (overwhelmingly MPI, which scales to large 
systems).  The hardest part of an SPMD program is designing the data 
layout and synchronizing the processors.  For a Fortran programmer like 
me, MPI is still an awkward way to express the resulting design.  But 
that's the best option most people have at this point.

Cheers,
Bill

-- 
Bill Long                                   longb at cray.com
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