Semantics of Send Modes

order

messages are non-overtaking

   CALL mpi_comm_rank(comm, rank, ierr)
   IF (rank .EQ. 0 ) THEN
      CALL mpi_bsend(buf1, count, MPI_REAL, 1, tag, comm, ierr)
      CALL mpi_bsend(buf1, count, MPI_REAL, 1, tag, comm, ierr)
   ELSE  ! rank .EQ. 1
      CALL mpi_recv(buf1, count, MPI_REAL, 0, MPI_ANY_TAG, comm, status, ierr)
      CALL mpi_recv(buf2, count, MPI_REAL, 0, tag, comm, status, ierr)
   END IF

progress

if a pair of matching send and receives have been initiated on two processes, then at least one of these two operations will complete, independently of other actions in the system:

   CALL mpi_comm_rank(comm, rank, ierr)
   IF (rank .EQ. 0) THEN
      CALL mpi_bsend(buf1, count, MPI_REAL, 1, tag1, comm, ierr)
      CALL mpi_ssend(buf2, count, MPI_REAL, 1, tag2, comm, ierr)
   ELSE    ! rank .EQ. 1
      CALL mpi_recv(buf1, count, MPI_REAL, 0, tag2, comm, status, ierr)
      CALL mpi_recv(buf2, count, MPI_REAL, 0, tag1, comm, status, ierr)
   END IF

fairness

there is no guarantee of fairness in handling communication

resource limitations

any pending communication consumes system resources if they are limited - errors will result if resources are not available. Users can attach their own buffers with

MPI_BUFFER_ATTACH( BUFFER, SIZE, IERROR )

and detach them with

MPI_BUFFER_DETACH( BUFFER_ADDR, SIZE, IERROR )

deadlocks

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example 1

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CALL mpi_comm_rank( comm, rank, ierr )
IF ( rank .EQ. 0 ) THEN
   CALL mpi_send(sendbuf, count, MPI_REAL, 1, tag, comm, ierr)
   CALL mpi_recv(recvbuf, count, MPI_REAL, 1, tag, comm, status, ierr)
ELSE    ! rank .EQ. 1
   CALL mpi_recv(recvbuf, count, MPI_REAL, 0, tag, comm, status, ierr)
   CALL mpi_send(sendbuf, count, MPI_REAL, 0, tag, comm, ierr)
END IF

this program will succeed even if no buffer is available. Standard SENDs can be replaced with synchronous ones.

example 2

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CALL mpi_comm_rank( comm, rank, ierr )
IF ( rank .EQ. 0 ) THEN
   CALL mpi_recv(recvbuf, count, MPI_REAL, 1, tag, comm, status, ierr)
   CALL mpi_send(sendbuf, count, MPI_REAL, 1, tag, comm, ierr)
ELSE    ! rank .EQ. 1
   CALL mpi_recv(recvbuf, count, MPI_REAL, 0, tag, comm, status, ierr)
   CALL mpi_send(sendbuf, count, MPI_REAL, 0, tag, comm, ierr)
END IF

This program will always deadlock, for all SEND modes.

example 3

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CALL mpi_comm_rank( comm, rank, ierr )
IF ( rank .EQ. 0 ) THEN
   CALL mpi_send(sendbuf, count, MPI_REAL, 1, tag, comm, ierr)
   CALL mpi_recv(recvbuf, count, MPI_REAL, 1, tag, comm, status, ierr)
ELSE    ! rank .EQ. 1
   CALL mpi_send(sendbuf, count, MPI_REAL, 0, tag, comm, ierr)
   CALL mpi_recv(recvbuf, count, MPI_REAL, 0, tag, comm, status, ierr)
END IF

This program will succeed only if communication system can buffer at least count reals.