I'm looking at replacing/modernizing an old application-specific MPI layer that was written around the turn of the century (that sounds odd). A major part of it is a mesh subdomain "halo exchange" for domain decomposition. When I dug into the implementation I was a little surprised to see it used point-to-point communication with ISend/Recv and rank-randomized issues of the sends (for better performance?) rather than Alltoallv, which I think would have been a more straightforward alternative (but my MPI understanding is limited). Some questions:
1) It seems that now defining a virtual topology and using Neighbor_alltoallv is a perfect match for this problem. Is there any reason today to not prefer this over individual send/recv? 2) I'm baffled about what I'm supposed to with the possible reordering of ranks that MPI_Dist_Graph_create_adjacent does. I understand the benefit of the communication pattern between ranks being matched to the underlying hardware topology, however the processes are already pinned (?) to specific cores, so I'm not sure what the relevance is of the assigned rank -- it's just a label, no? Or am I expected to migrate my data; e.g. if old-com rank p becomes new-com rank q, am I supposed to migrate the data from the old rank p to old rank q before using the new com? Thanks!
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