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Problems with M things

Problems with M things. Review: NULL MPI handles. All MPI_*_NULL are handles to invalid MPI objects MPI_REQUEST_NULL is an exception Calling MPI_TEST/MPI_WAIT with REQUEST_NULL results in the “empty” status status.MPI_SOURCE = MPI_ANY_SOURCE s tatus.MPI_TAG = MPI_ANY_TAG

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Problems with M things

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  1. Problems with M things

  2. Review: NULL MPI handles • All MPI_*_NULL are handles to invalid MPI objects • MPI_REQUEST_NULL is an exception • Calling MPI_TEST/MPI_WAIT with REQUEST_NULL results in the “empty” status • status.MPI_SOURCE = MPI_ANY_SOURCE • status.MPI_TAG = MPI_ANY_TAG • status count = zero • …otherwise the ANY/SOME array-based TEST/WAIT functions would be kind of a disaster

  3. Review: PROC_NULL behavior • Call MPI_RECV with MPI_PROC_NULL • Get “PROC_NULL status” • Same as “empty status”, but with MPI_SOURCE=MPI_PROC_NULL • Call MPI_IRECV with MPI_PROC_NULL • Get “PROC_NULL” status • (…as expected)

  4. MPI_MESSAGE_NULL: MRECV • Defining PROC_NULL behavior with MPROBE • MPI-3 allows calling MPI_[I]MRECV with MPI_MESSAGE_NULL • You get “PROC_NULL” status back MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NULL MPI_MRECV(…, &msg, &status) Get status=“PROC_NULL” status

  5. MPI_MESSAGE_NULL: MRECV ✔ • Defining PROC_NULL behavior with MPROBE • MPI-3 allows calling MPI_[I]MRECV with MPI_MESSAGE_NULL • You get “PROC_NULL” status back MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NULL ✖ MPI_MRECV(…, &msg, &status) We’re calling MPI_MRECV with an invalid message! ✔ Get status=“PROC_NULL” status

  6. MPI_MESSAGE_NULL: IMRECV • Defining PROC_NULL behavior with MPROBE • MPI-3 allows calling MPI_[I]MRECV with MPI_MESSAGE_NULL • You get “PROC_NULL” status back MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NULL MPI_IMRECV(…, &msg, &req) MPI_WAIT(&req, &status) Get status=“PROC_NULL” status

  7. MPI_MESSAGE_NULL: IMRECV ✔ • Defining PROC_NULL behavior with MPROBE • MPI-3 allows calling MPI_[I]MRECV with MPI_MESSAGE_NULL • You get “PROC_NULL” status back MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NULL ✖ MPI_IMRECV(…, &msg, &req) MPI_WAIT(&req, &status) We’re calling MPI_IMRECV with an invalid message! ✔ Get status=“PROC_NULL” status

  8. The Problem • Overloading MPI_MESSAGE_NULL: • Invalid message, and • MPI_PROC_NULL message • FAIL

  9. Solution: MPI_MESSAGE_NO_PROC • When [I]MPROBE with PROC_NULL • Get msg = MPI_MESSAGE_NO_PROC MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NO_PROC MPI_MRECV(…, &msg, &status) Set msg=MPI_MESSAGE_NULL Get status=“PROC_NULL” status

  10. Rationale • Consistency: MPI_*_NULL are invalid handles • Can avoid calling MPI_MRECV (or MPI_IMRECV) if desired • Same solution applies to both MRECV and IMRECV • Alternate names: • MPI_MESSAGE_PROC_NULL (BAD) MPI_MPROBE(MPI_PROC_NULL, …, &msg, …) Get msg=MPI_MESSAGE_NO_PROC MPI_MRECV(…, &msg, &status) Set msg=MPI_MESSAGE_NULL Get status=“PROC_NULL” status

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