[MPI3 Fortran] MPI Data types

[Jeff Squyres]

On May 7, 2009, at 1:30 PM, Jeff Squyres wrote:

> This brings up the struggle I brought before: why does Fortran need  
> the (count, MPI_Datatype) tuple?  Because it's part of MPI, that's  
> why -- it's the standard.

I let out an important piece of rationale here: part of the whole  
reason to have MPI_Datatypes is to allow the MPI implementation to  
optimize the memory layout map *once*, and then just use that  
optimized map forevermore.  If you have no MPI_Datatype argument (that  
is a handle to this optimized map), then you have to construct  
datatypes (and optimize them) on the fly.  This has shown, in real- 
world MPI codes, to be quite expensive (read: slow).

-- 
Jeff Squyres
Cisco Systems