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CS 420

CS 420. Overview of MPI MPI Concepts Basic MPI Functions. MPI Overview. MPI = Message Passing Interface A mechanism/protocol/standard for developing and implementing parallel programs. MPI. MPI evolved from other message passing systems Other message passing systems include-

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CS 420

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  1. CS 420 Overview of MPI MPI Concepts Basic MPI Functions

  2. MPI Overview • MPI = Message Passing Interface • A mechanism/protocol/standard for developing and implementing parallel programs

  3. MPI • MPI evolved from other message passing systems • Other message passing systems include- • PICL, Chameleon, Zipcode, TCGMSG • Notably PVM (Parallel Virtual Machine) • MPI was intended as the successor to PVM

  4. MPI • MPI is not a language • MPI is a library • Can be called from and used by C, C++ and Fortran

  5. MPI • Some advantages of MPI • Standard • MPI implementations tend to stick to the standard • Portable • Versatile • Performance • Small core set of functions

  6. MPI • MPI has a number of flavors (implementations) • MPICH • LAM/MPI • Chimp • MPICH-G (Globus) • MPICH-MX (Myricom Express) • MVICH

  7. MPI • A number of process managers • mpirun • mpiexec • mpd • lam • mpe

  8. MPI Concepts • Messages – packets of data – might be scalar, arrays, strings, structures. • No guarantee regarding the ordering of messages- nondeterministic • Processes vs Processors • Process Ids • Ranks • Blocking and Non-blocking

  9. Five Processor DMC R1 array(1),array(2),… R2 R3 x, y, z, temp,… R4 R5

  10. MPI Concepts • Source and Destination • Can have wildcards • Tags • Arbitrary message ids … but • Be careful

  11. MPI Concepts • Groups • Communicators • MPI_WORLD_COMM – global communicator • MPI Data Types • MPI_INT

  12. Basic MPI functions • … but first include… • #include “mpi.h”

  13. Basic MPI Functions • int MPI_Init(int argc_ptr, char** argv_ptr) • Initializes MPI in the application • MPI_Init(&argc, &argv); • int MPI_Finalize() • Closes/shutdown MPI for the application • MPI_Finalize();

  14. Basic MPI Functions • int MPI_Comm_size(MPI_comm comm, int* size) • Returns the size of the group in size • MPI_Comm_size(MPI_COMM_WORLD, &nprocs) • Int MPI_Comm_rank(MPI_comm comm, int* rank) • Returns the rank (ID) of this process in rank • MPI_Comm_rank(MPI_WORLD_COMM, &myrank);

  15. Basic MPI Functions … So a program might look like this… #include <stdio.h> #include “mpi.h” main(int argc, char* argv[]) { int size; int myrank; MPI_Init(&argc, &argv); MPI_Comm_size(MPI_COMM_WORLD, &size ); MPI_Comm_rank(MPI_COMM_WORLD, &myrank); {other stuff here} MPI_Finalize(); }

  16. Other useful functions • double MPI_Wtime(void) • Returns the walltime from some predetermined point • double starttime = MPI_Wtime(); • double MPI_Wtick(void) • Returns the precision of the timer • double precision = MPI_Wtick(); • Note: times on processes are independent

  17. Other MPI information • LLNL Tutorial • http://www.llnl.gov/computing/tutorials/mpi/ • Argonne National Laboratories • http://www-unix.mcs.anl.gov/mpi/ • The MPI Forum • http://www.mpi-forum.org/ • MPI Book • http://www.netlib.org/utk/papers/mpi-book/mpi-book.html • Real Books – Pacheco, Gropp et. al.

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