MPJ Express Sending and Receiving - ClassCastException

Question

I've looked at this but it didn't reallly help as I'm dealing with primitives, not objects. This is just part of a extra section is a project. It's fine as it is, but I'd like to have it working.

It's compiling fine, but when I run it, I get, at line 20, a ClassCastException.

/* Java Version (mine) */    
public class PingPongJava { 

    public static void main(String args[]) throws Exception { 
        int me;
        int size;
        int recv_buf = 0;
        int buf = 0;
        int send_buf = 101;
        double tic = 0, toc = 0;

        MPI.Init(args);
        me = MPI.COMM_WORLD.Rank();
        size = MPI.COMM_WORLD.Size();

        tic = MPI.Wtime();
        for (int i = 0; i < 50; i++){
            if (me == 0){
            MPI.COMM_WORLD.Send(send_buf, 0,1,MPI.INT, 17, 100);
            MPI.COMM_WORLD.Recv(recv_buf, 0,1, MPI.INT, 23, 100);

            }
            else{
            MPI.COMM_WORLD.Recv(buf, 0, 1, MPI.INT, 17, 100);
            MPI.COMM_WORLD.Send(buf, 0,1,MPI.INT, 23, 100);
            }
        }
        toc = MPI.Wtime();
        if (me == 0){
            System.out.println("Time taken is " + (toc-tic)/100);
            }

        MPI.Finalize(); 
     }
    }

I'm converting from this C version

/* Point-to-point communication.
 */
#include <mpi.h>
#include <stdio.h>

int main(int argc, char **argv){
  int rank, size, i, recv_buf, send_buf=101, buf;
  MPI_Status status;
  double tic, toc;

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

  tic = MPI_Wtime();
  for(i=0;i<50;i++){
    if(rank==0){
      MPI_Send(&send_buf, 1, MPI_INT, 1, 17 , MPI_COMM_WORLD);
      MPI_Recv(&recv_buf, 1, MPI_INT, 1, 23, MPI_COMM_WORLD, &status);
    }else{
      MPI_Recv(&buf, 1, MPI_INT, 0, 17, MPI_COMM_WORLD, &status);
      MPI_Send(&buf, 1, MPI_INT, 0, 23, MPI_COMM_WORLD);
    }
  }
  toc = MPI_Wtime();

  if(rank==0)
    printf("Average time for a single message: %lf seconds \n", (toc-tic)/100.0);

  MPI_Finalize();
  return 0;
}

score 1 · Answer 1 · answered Jun 14 '14 at 10:10

Firstly, you must use array type for variables recv_buf, buf, send_buf. Secondary, your two last arguments in Send, Recv signatures are wrong. They must be: "..destination, tag)" in Send and "..source, tag)" in Receive. I corrected yours mistakes in code below.

public class PingPongJava {

    public static void main(String args[]) throws Exception {
        int me;
        int size;
        int[] recv_buf = { 0 };
        int[] buf = {0};
        int[] send_buf = { 101 };
        double tic = 0, toc = 0;

        MPI.Init(args);

        me = MPI.COMM_WORLD.Rank();
        size = MPI.COMM_WORLD.Size();

        tic = MPI.Wtime();

        for (int i = 0; i < 50; i++) {
            if (me == 0) {
                MPI.COMM_WORLD.Send(send_buf, 0, 1, MPI.INT, 1, 17);
                MPI.COMM_WORLD.Recv(recv_buf, 0, 1, MPI.INT, 1, 23);
            } else {
                MPI.COMM_WORLD.Recv(buf, 0, 1, MPI.INT, 0, 17);
                MPI.COMM_WORLD.Send(buf, 0, 1, MPI.INT, 0, 23);
            }
        }
        toc = MPI.Wtime();

        if (me == 0) {
            System.out.println("Time taken is " + (toc - tic) / 100);
        }

        MPI.Finalize();
    }
}

MPJ Express Sending and Receiving - ClassCastException

1 Answers1