Current Message-Passing

Message = data + envelope

Figure 3.4: Data+Envelope.

• MPI Data; Arguments
  • startbuf (starting location of data)
  • count (number of elements)
    • receive count $\geq$ send count
  • datatype (basic or derived)
    • receiver datatype = send datatype (unless MPI_PACKED)
    • Elementary (all C and FORTRAN types). Specifications of elementary datatypes allows heterogeneous communication.
    • MPI basic datatypes for C:

      Figure 3.5: MPI basic datatypes for C.

      
• MPI Envelope; Arguments
  • destination or source
    • rank in a communicator
    • receive = sender or MPI_ANY_SOURCE
  • tag
    • integer chosen by programmer
    • receive = sender or MPI_ANY_TAG (wild cards allowed)
  • communicator
    • defines communication "space"
    • group + context
    • receive = send
  • Collective operations typically operated on all processes.
  • All communication (not just collective operations) takes place in groups.
  • A context partitions the communication space. A message sent in one context cannot be received in another context. Contexts are managed by the system.
  • A group and a context are combined in a communicator.
  • Source/destination in send/receive operations refer to rank in group associated with a given communicator.