[Mpi-forum] large count support not as easy as people seem to have thought

[Rob Latham]

On 05/06/2014 12:19 PM, Jeff Hammond wrote:
> Issue #2: cannot use built-in reduce ops.
>
> Once we rule out using built-in ops with our large-count datatypes, we
> must reimplement all of the reduction operations required.  I find
> this to be nontrivial.  I have not yet figured out how to get at the
> underlying datatype info in a simple manner.  It appears that
> MPI_Type_get_envelope exists for this purpose, but it's a huge pain to
> have to call this function when all I need to know is the number of
> built-in datatypes so that I can apply my clever and use
> MPI_Reduce_local inside of my user-defined operation.

To determine the number of built-in datatypes, yes, one must recursively 
call MPI_Type_get_envelope and MPI_Type_get_contents.  See the ROMIO 
datatype flattening code for just how much of a pain 
MPI_Type_get_contents and MPI_Type_get_envelope is.

You might find Rob Ross's libmpitypes approach a bit better, but that 
code is designed for applying functions to the data of a datatype, 
deliberately bypassing the get_envelope/get_contents information.

==rob

-- 
Rob Latham
Mathematics and Computer Science Division
Argonne National Lab, IL USA