Message Passing

• Tasks, in Unix generally separate processes, are complete programs with their own local data.

  • Each task has its own ID.
  • Tasks communicate by sending and receiving messages.

• Widely available message passing libraries:

  • PVM
    • good for clusters.
  • MPI
    • emerging standard.

#include <stdio.h>
#include "mpi.h"
main(int argc,char **argv)
{
    int rank,size,sdata=100155,rdata=0;
    MPI_Status status;
    MPI_Init(&argc,&argv);
    MPI_Comm_rank( MPI_COMM_WORLD, &rank);
    MPI_Comm_size( MPI_COMM_WORLD, &size);
    if(rank==0) {
        MPI_Send(&sdata,1,MPI_INT,1,0,
                 MPI_COMM_WORLD);
        MPI_Recv(&rdata,1,MPI_INT,1,1,
                 MPI_COMM_WORLD,&status);
        if(rdata==sdata)
             printf("test succeeds\n");
        else printf("test fails\n");
    }
    else {
        MPI_Recv(&rdata,1,MPI_INT,0,0,
                  MPI_COMM_WORLD,&status);
        MPI_Send(&rdata,1,MPI_INT,0,1,
                  MPI_COMM_WORLD);
    }
    MPI_Finalize(); exit(0);
}