What's a communicator?

Overview

Teaching: 10 min
Exercises: 0 min

Questions

Objectives

What the code does

• (FORTRAN version; C is similar)

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

• use mpi : imports declarations for MPI function calls
• call MPI_INIT(ierr): initialization for MPI library. Must come first. ierr: Returns any error code.
• call MPI_FINALIZE(ierr): close up MPI stuff. Must come last. ierr: Returns any error code.
• call MPI_COMM_RANK, call MPI_COMM_SIZE: requires a little more explanation.

Communicators

• MPI groups processes into communicators.
• Each communicator has some size – number of tasks.
• Each task has a rank 0..size-1
• Every task in your program belongs to MPI_COMM_WORLD

MPI_COMM_WORLD: size=4, ranks=0..3

• Can create our own communicators over the same tasks
• May break the tasks up into subgroups
• May just re-order them for some reason

• So in this program:

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

• we call MPI_COMM_RANK, call MPI_COMM_SIZE …
• to get the size of communicator, and this tasks’s rank within communicator.
• put answers in rank and size

Rank and Size much more important in MPI than OpenMP

• In OpenMP, compiler assigns jobs to each thread; don’t need to know which one you are.
• MPI: processes determine amongst themselves which piece of puzzle to work on, then communicate with appropriate others.

Let’s look at the same program 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;
}

• #include<mpi.h> vs use mpi
• C - functions return ierr;
• Fortran - pass ierr
• Difference in parameters to MPI_Init

Key Points