• Message Passing Interface standard

• The first standard and portable message passing library with good performance

• "Standard" by consensus of MPI Forum participants from over 40 organizations

• Finished and published in May 1994, updated in June 1995

• MPI 2 now being worked on. Extends (does not change) MPI. Expected mid-1997.

• Document that defines MPI is on the World Wide Web at Argonne National Lab

1.4 How to Use MPI

• If starting from serial version, make sure thoroughly debugged

• If program is new, but you can easily begin with a serial version, do so, to get it thoroughly debugged

• Design parallel algorithm

• Write code, making calls to MPI library

• In routines that call MPI, use header file:
  #include "mpi.h" for C programs
  include 'mpif.h' for Fortran programs
  

• Compile and run using IBM's Parallel Environment
  (covered in the IBM SP Parallel Environment module)

• Run with a few nodes first, increase number gradually

2.1 Format of MPI calls

• C bindings
  • rc = MPI_Xxxxx(parameter, ... )
  • returns integer error code, which is MPI_SUCCESS if successful

• Fortran bindings
  • call MPI_XXXXX(parameter,..., ierror)
  • case insensitive
  • returns integer error code ierror, =MPI_SUCCESS if successful

• Exception: timing functions return double precision reals

• MPI has over 125 functions, but you can do much with just 6:
  • Initialize for communications
    MPI_INIT: initializes for the MPI environment
    MPI_COMM_SIZE: returns the number of processes
    MPI_COMM_RANK: returns this process's number (rank)
    
  • Do the communications necessary for coordinating computation
    MPI_SEND: sends a message
    MPI_RECV: receives a message
    
  • Exit in a "clean" fashion when done communicating
    
    MPI_FINALIZE
    

Click here for C version

   program hello
   include 'mpif.h'
   integer rank, size, ierror, tag, status(MPI_STATUS_SIZE)
   character(12) message
	
   call MPI_INIT(ierror)
   call MPI_COMM_SIZE(MPI_COMM_WORLD, size, ierror)
   call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierror)
   tag = 100

   if (rank .eq. 0) then
      message = 'Hello, world'
      do i=1, size-1
         call MPI_SEND(message, 12, MPI_CHARACTER, i, tag,
  &                    MPI_COMM_WORLD, ierror)
      enddo

   else
    	 call MPI_RECV(message, 12, MPI_CHARACTER, 0, tag,
  &                    MPI_COMM_WORLD, status, ierror)
   endif

   print*, 'node', rank, ':', message
   call MPI_FINALIZE(ierror)
   end

 call MPI_SEND(startbuf,count,datatype,
                   \     |    /
                    ---DATA--- 

                 dest,tag,comm,  ierror)
                   \   |   /
                    ENVELOPE

• Arguments
  • startbuf (starting location of data)
  • count (number of elements)

    receive >= send

  • datatype (basic or derived)

    receiver = send (unless MPI_PACKED)

• MPI basic datatypes for Fortran

    ---------------------------------------
    MPI Datatype           Fortran Datatype  
    ---------------------------------------
    MPI_INTEGER            INTEGER           
    MPI_REAL               REAL              
    MPI_DOUBLE_PRECISION   DOUBLE PRECISION  
    MPI_COMPLEX            COMPLEX           
    MPI_LOGICAL            LOGICAL           
    MPI_CHARACTER          CHARACTER(1)      
    MPI_BYTE                                 
    MPI_PACKED                               
    ---------------------------------------
  

  • C datatypes

• Derived datatypes
  • mixed datatypes
  • non-contiguous data

• Destination or source
  • rank in a communicator
  • receive = sender or MPI_ANY_SOURCE
• Tag
  • integer chosen by programmer
  • receive = sender or MPI_ANY_TAG
• Communicator
  • defines communication "space"
  • receive = send
• Analogy: bill collection agency

4.1 Why have communicators?

• If you are writing a library, can you choose safe tags?
• Example: variable (and possibly incorrect) behavior if no communicator

Animation

• Programmers can define a process group
• And define new communicator(s) for the process group

• MPI program: 6 basic calls
  • MPI_INIT
  • MPI_COMM_RANK
  • MPI_COMM_SIZE
  • MPI_SEND
  • MPI_RECV
  • MPI_FINALIZE

• MPI messages
  • data (startbuf, count, datatype)
  • envelope (destination/source, tag, communicator)

• Communicators

Message Passing Interface Forum (1995) MPI: A Message Passing Interface Standard. June 12, 1995. Available online or in postscript.

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