[MPI3 Fortran] MPI Data types
jsquyres at cisco.com
Thu May 7 12:34:00 CDT 2009
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).
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