Communicate data with `count` value close to `INT_MAX`

Question

The Message Passing Interface APIs always use int as a type for count variables. For instance, the prototype for MPI_Send is:

Answer 1

The MPI Forum is extremely reluctant to making major changes to the MPI API in order to introduce 64-bit support. The reason for that is both maintaining backward compatibility and not introducing seldom used features - it appears to be an almost as vigorous process as the one that keeps Fortran 2xxx at large compatible with prehistoric FORTRAN IV programs.

As evident by the ticket, creating a large datatype to work around the limitation is actually viewed as not so hackish solution by many, even by William D. Gropp himself:

First, it is possible to send much larger data by simply creating an appropriate MPI Datatype (this could be easier, but it is possible). Second, sending such large data will take seconds (at least!) on current platforms (8GB just for 4-byte integers and a 2GB count) - so this should not be a common operation (and the overhead of creating and committing and freeing a datatype should be negligible).

The fact that MPI-3.0 introduced official support for building large (more than 2³¹ elements) datatypes while the proposal to change the count argument of calls like MPI_SEND to MPI_Count / INTEGER(KIND=MPI_COUNT_KIND) was rejected should hint you on the way of thinking that prevails the MPI Forum. Even before MPI-3.0, 64-bit internal sizes were used by some implementations since years (e.g. Open MPI) while others have chosen to remain on the 32-bit bandwagon (e.g. Intel MPI).