Hello, MPI World!

Overview

Teaching: 10 min
Exercises: 15 min

Questions

Objectives

MPI is a Library for Message-Passing

• Not built in to compiler
• Function calls that can be made from any compiler, many languages
• Just link to it
• Wrappers: mpicc, mpif77

Below program is written in c.

#include <stdio.h>
#include <mpi.h>

int main(int argc, char **argv) {
    int rank, size;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    printf("Hello, World, from task %d of %d\n",rank,  size);

    MPI_Finalize();
    return 0;
}

Below program is written in fortran.

program helloworld
    use mpi
    implicit none
    integer :: rank, comsize, ierr

    call MPI_Init(ierr)
    call MPI_Comm_size(MPI_COMM_WORLD, comsize, ierr)
    call MPI_Comm_rank(MPI_COMM_WORLD, rank, ierr)

    print *,'Hello World, from task ', rank, 'of', comsize

    call MPI_Finalize(ierr)
end program helloworld

MPI is a Library for Message-Passing

• Communication/coordination between tasks done by sending and receiving messages.
• Each message involves a function call from each of the programs.

• Three basic sets of functionality:
  • Pairwise communications via messages
  • Collective operations via messages
  • Efficient routines for getting data from memory into messages and vice versa

Messages

• Messages have a sender and a receiver
• When you are sending a message, don’t need to specify sender (it’s the current processor),
• A sent message has to be actively received by the receiving process

• MPI messages are a string of length count all of some fixed MPI type
• MPI types exist for characters, integers, floating point numbers, etc.
• An arbitrary integer tag is also included - helps keep things straight if lots of messages are sent.

Size of MPI Library

• Many, many functions (>200)
• Not nearly so many concepts
• We’ll get started with just 10-12, use more as needed.

MPI_Init()  
MPI_Comm_size()  
MPI_Comm_rank()  
MPI_Ssend()  
MPI_Recv()  
MPI_Finalize()  

Hello World

• The obligatory starting point

C

#include <stdio.h>
#include <mpi.h>

int main(int argc, char **argv) {
    int rank, size;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    printf("Hello, World, from task %d of %d\n",rank,  size);

    MPI_Finalize();
    return 0;
}

Fortran

program helloworld
    use mpi
    implicit none
    integer :: rank, comsize, ierr

    call MPI_Init(ierr)
    call MPI_Comm_size(MPI_COMM_WORLD, comsize, ierr)
    call MPI_Comm_rank(MPI_COMM_WORLD, rank, ierr)

    print *,'Hello World, from task ', rank, 'of', comsize

    call MPI_Finalize(ierr)
end program helloworld

Compile and run it

Pick a language and type in the code. You’ll learn more if you don’t cut and paste! Compile it with mpicc or mpif90 then run the result with mpirun.

Solution

$ cd mpi/mpi-intro

$ mpif90 hello-world.f90 -o hello-world
or 
$ mpicc hello-world.c -o hello-world

$ mpirun -np 1 hello-world
$ mpirun -np 2 hello-world
$ mpirun -np 8 hello-world

Key Points