Jonathan Carroll-Nellenback CIRC Summer School MESSAGE PASSING INTERFACE (MPI)

 MPI – message passing interface  Language independent communications protocol  MPI-1, MPI-2, and MPI-3 standards  Implementations typically consist of a specific set of routines callable from C, C++, or Fortran  MPICH (implements MPI – 3)  Open MPI – 1.8 (Implements MPI – 2)  IMPI – Intel MPI  MVAPICH,  Commercial implementations  Bindings for Python, Java, etc... BACKGROUND

 First MPI Program  /public/jcarrol5/mpi/example1.f90  MPI_Init – initializes mpi  MPI_Comm_rank – gets the current task rank within the communicator (starting at 0)  MPI_Comm_size – gets the size of the communicator  MPI_Finalize – closes mpi  MPI_Reduce – Collective communication  MPI_Allreduce – Collective communication  Compiling  module load openmpi  mpif90 example1.f90 (mpicc example1.c)  srun –p debug –n 4 –o output_%t a.out OUTLINE

 Parallelize exercise1.f90 or exercise1.c EXERCISE 1

 /public/jcarrol5/mpi/example2.f90  1 to All  MPI_Bcast – Broadcasts the same data to all ranks  MPI_Scatter – Evenly Distributes data to all ranks  MPI_Scatterv – Unevenly distributes data to all ranks  All to 1  MPI_Reduce – Performs a reduction operation towards a single rank  MPI_Gather – Collects evenly distributed data on one rank  MPI_Gatherv – Collects unevenly distributed data on one rank  All to All  MPI_Allreduce – Performs a reduction and broadcasts the result  MPI_Allgather – Collects evenly distributed data onto all ranks  MPI_Allgatherv – Collects unevenly distributed data onto all ranks  MPI_Alltoall – Scatter/Gather  MPI_Alltoallv – Scatterv/Gatherv COLLECTIVE COMMUNICATION ROUTINES

 Reduction Operations – MPI_MAX, MPI_MIN, MPI_SUM, MPI_PROD, MPI_BAND, MPI_BOR, MPI_BXOR, MPI_LAND, MPI_LOR, MPI_LXOR  C Data types – MPI_CHAR, MPI_SHORT, MPI_INT, MPI_LONG, MPI_UNSIGNED_CHAR, MPI_UNSIGNED_SHORT, MPI_UNSIGNED, MPI_UNSIGNED_LONG, MPI_FLOAT, MPI_DOUBLE, MPI_LONG_DOUBLE, MPI_BYTE, MPI_PACKED  Fortran Data types – MPI_CHARACTER, MPI_INTEGER, MPI_REAL, MPI_LOGICAL, MPI_INTEGER1, MPI_INTEGER2, MPI_INTEGER4, MPI_REAL2, MPI_REAL4, MPI_REAL8, MPI_DOUBLE_PRECISION, MPI_COMPLEX, MPI_DOUBLE_COMPLEX, MPI_BYTE, MPI_PACKED IMPORTANT CONSTANTS

 Parallelize exercise2.f90 or exercise2.c EXERCISE 2

 /public/jcarrol5/mpi/example3.f90  Tags – additional identifiers on messages  MPI_Send  MPI_Recv BASIC SENDING AND RECEIVING

 Modify your program from exercise2 to not use any global communication routines. EXERCISE 3

 Blocking vs Non-blocking  Non Blocking sends and receives will immediately return control to the calling routine. However, they usually will require buffering and testing later on to see whether the send/recv has completed.  Good for overlapping communication with computation  May lead to extra buffering  Synchronous vs Asynchronous  Synchronous sends require a matching recv to be called before returning. Blocking only if recv has not been posted. Does not require any additional buffering.  Buffered vs NonBuffered  Buffered sends explicitly buffer the data to be sent so that the calling routine can release the memory.  Ready send  Assumes that the receiver has already posted the recv. SENDING MODES

 /public/jcarrol5/mpi/example4.f90  MPI_Send – May or may not block  MPI_Bsend – May buffer – returns immediately  MPI_Ssend – Synchronous Send (returns after matching recv posted)  MPi_Rsend – Ready send (matching recv must be posted)  MPI_Isend – Nonblocking send (must check for completion)  MPI_Ibsend – Nonblocking buffered send  MPI_Issend – Nonblocking synchronous send  MPI_Irsend- Nonblocking ready send  MPI_Recv – Blocking receive  MPI_IRecv – Nonblocking receive SEND ROUTINES

 Rewrite exercise 3 using ready sends(rsend), synchronous sends (ssend), and nonblocking sends (isend) and see if it is any faster. EXERCISE 5

 /public/jcarrol5/mpi/example5.f90  MPI starts with one communicator (MPI_COMM_WORLD)  Separate communicator groups can be formed using  MPI_Comm_split  Or you can extract the group belonging to mpi_comm_world and create subgroups through various routines.  Multiple communicators can use the same group. COMMUNICATORS AND GROUPS