This came up recently and I believe it is allowed to wait/test a send request before all partitions are marked as ready. It's not explicitly stated, rather there is no restriction disallowing this usage. It may be good to clarify this semantic (e.g. in Section 4.2.2) for MPI 4.1 since this is a topic that the WG debated at length. @Ryan Grant <[email protected]> Please fact check. ~Jim. On Mon, Oct 10, 2022 at 7:22 AM Joachim Jenke via mpiwg-persistence < [email protected]> wrote:
Hello wg-persistence,
Looking at the MPI 4.0 document, it is not clear to us, whether it is allowed to call a completion call for a partitioned communication request before all partitions are marked ready. A simple single-threaded example would be:
```C MPI_Psend_init(message, partitions, COUNT, MPI_DOUBLE, dest, tag, MPI_COMM_WORLD, MPI_INFO_NULL, &request); MPI_Start(&request); for(i = 0; i < partitions-1; ++i) { MPI_Pready(i, request); } MPI_Test(&request, &flag, MPI_STATUS_IGNORE); // flag will always be 0 MPI_Pready(partitions-1, request); MPI_Wait(&request, MPI_STATUS_IGNORE); MPI_Request_free(&request); ```
The question becomes more relevant in a multi-threaded context. One thread could finish the work early and call MPI_Wait to detect when all partitions were sent.
From my understanding, the only requirement is that all partitions must be marked ready with explicit ready calls before the operation can complete. Replacing the test in above example with a wait call would result in deadlock.
Best Joachim
-- Dr. rer. nat. Joachim Jenke
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