Re: [OMPI users] Any scientific application heavily using MPI_Barrier?

[Eugene Loh]

Going back to this thread from earlier this calendar year... Ganesh wrote: Hi Dick, Jeff paraphrased an unnamed source as suggesting that: "any MPI program that relies on a barrier for correctness is an incorrect MPI application." . That is probably too strong. How about this assertion? If there are no wildcard receives - every MPI_Barrier call is semantically irrelevant. This depends on what 'semantically irrelevant' means.  It is clear that one can write a wildcard-free program that will deadlock if you insert a barrier incorrectly, but that removing the barrier will avoid the deadlock. (Imagine P1 doing a Send; Barrier and P2 doing a Barrier; Receive(nonwildcard)). So a wildcard-free program may still deadlock (semantically noticeable effect) by having barriers. I'm sure you did not mean to include this degenerate nit-pick - but yes otherwise you are right! And similarly (to disprove Jeff's original assertion), if there are wildcards, then an MPI_Barrier call could be semantically relevent. E.g., let's say P0 and P1 each send a message to P2, both using the same tag and communicator.  Let's say P2 does two receives on that communicator and tag, using a wildcard source.  So, the messages could be received in either order.  One could introduce barriers to order the messages.  E.g., P0:   Send   Barrier P1:   Barrier   Send P2:   Recv   Barrier   Recv