TotalView MPI Exercise 1

Assignment

Your assignment is to locate an error in the following program. The program is hanging before it gets to the print statement. Compile the code with ifort and locate the source of the error.

#include <stdio.h>
#include "mpi.h"
int main (int argc, char **argv)
{
     int rank, size, isbuf, ierr;
     int itag, myrank, irbuf;
     MPI_Init( &argc, &argv );
     MPI_Comm_rank( MPI_COMM_WORLD, &rank );
     MPI_Comm_size( MPI_COMM_WORLD, &size );
     MPI_Status status;
     isbuf=1;
     itag=1;
     if (myrank == 0){
       while (isbuf != 10) {
         isbuf+=2;
       }
       MPI_Send(&isbuf,1,MPI_INT,1,itag,MPI_COMM_WORLD);
     }
     else {
       MPI_Recv(&irbuf,1,MPI_INT,0,itag,MPI_COMM_WORLD,&status);
       printf(" I RECEIVED isbuf =%d", irbuf);
     }
     MPI_Finalize();
     return 0;
}

Details

cut and paste the program into send.c

icc -g -lmpi -o send send.c

mpirun -np 2 ./send

This program will hang and produce no output. Press Control-C to kill the job.

Tips

Launch totalview with:
totalview mpirun -a -np 2 ./send
Click the button "Go"
When asked if you want to stop the job, respond "Yes"
Using totalview to set the breakpoint on either process 1 or process 2 to see till which line the job is going and at which line the job hangs.
After finding the line at which line the job hangs examine the variables and arguments, which have been used till that line and analyze what can be the culprit to cause the hang.

A solution

In the loop that incriments isbuf, it should only incriment by 1 at a time instead of 2. Alternatively, the initial value of isbuf could be set to a small even number.

#include <stdio.h>
#include "mpi.h"
int main (int argc, char **argv)
{
     int rank, size, isbuf, ierr;
     int itag, myrank, irbuf;
     MPI_Init( &argc, &argv );
     MPI_Comm_rank( MPI_COMM_WORLD, &rank );
     MPI_Comm_size( MPI_COMM_WORLD, &size );
     MPI_Status status;
     isbuf=0;
     itag=1;
     if (myrank == 0){
       while (isbuf != 10) {
         isbuf+=2;
       }
       MPI_Send(&isbuf, 1, MPI_INT, 1, itag,MPI_COMM_WORLD);
     }
     else {
        MPI_Recv( &irbuf, 1, MPI_INT, 0, itag,MPI_COMM_WORLD, &status);
        printf(" I RECEIVED isbuf =%d", irbuf);
     }
     MPI_Finalize();
     return 0;
}