Thanks. src/mpi/pt2pt/waitall.c has both code and comments indicating that it matters. The ideas for that code came from IBM (14f8143312e0dfd5eafcaedda8a874f66b912d1a), which presumably means that it helped on Blue Gene. It is easy enough to measure message rate with and without an array of status objects to understand this quantitatively. Best, Jeff On Fri, May 27, 2016 at 6:40 AM, William Gropp <[email protected]> wrote:

This was the motivation, more than avoiding declaring a variable that was

never read in the user’s program. There was evidence for this from Intel in the early ‘90s, though I don’t know if anyone has looked into it recently.

Bill

William Gropp Director, Parallel Computing Institute Thomas M. Siebel Chair in Computer Science Chief Scientist, NCSA University of Illinois Urbana-Champaign

On May 26, 2016, at 6:30 PM, Jeff Hammond <[email protected]> wrote:

Is it correct to assume that it is cheaper to check for MPI_STATUS_IGNORE

and MPI_STATUSES_IGNORE than to fill in the status object, in the case where the user does not need the status?

Thanks,

Jeff

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