On Thu, Feb 11, 2016 at 02:17:40PM +0000, Peter Wind wrote: > I would add that the present situation is bound to give problems for some > users. > It is natural to divide an array in segments, each process treating its > own segment, but needing to read adjacent segments too. > MPI_Win_allocate_shared seems to be designed for this. > This will work fine as long as no segment as size zero. It can also be > expected that most testing would be done with all segments larger than > zero. > The document adding "size = 0 is valid", would also make people confident > that it will be consistent for that special case too.
Nope, that statement says its ok for a rank to specify that the local shared memory segment is 0 bytes. Nothing more. The standard unfortunately does not define what pointer value is returned for a rank that specifies size = 0. Not sure if the RMA working group intentionally left that undefine... Anyway, Open MPI does not appear to be out of compliance with the standard here. To be safe you should use MPI_Win_shared_query as suggested. You can pass MPI_PROC_NULL as the rank to get the pointer for the first non-zero sized segment in the shared memory window. -Nathan HPC-5, LANL
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