Current Message-Passing

Message = data + envelope

Figure 2: 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)
    • derived datatype
      • mixed datatypes
      • contiguous arrays of datatypes
      • strided blocks of datatypes
      • indexed array of blocks of datatypes
      • general structure
    • Specifications of elementary datatypes allows heterogeneous communication.
    • MPI basic datatypes for C:

      Figure 3: 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.
  • MPI provides groups of processes
    • initial "all" group.
    • group management routines (build, delete groups).
  • 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.

All of these specifications are a good match to hardware, easy to understand, but too inflexible.