OpenMPI gets waiting for ever

Question

when i execute this code i have no response, it gets waiting for ever, process 1 is sending data to the root process but the root process doesn't receive it (or almost that is what i think).

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


void my_parallel_function(int v[], int size, int rank)
{
   MPI_Status status;
   int i;

   if(rank==0)
   {
     MPI_Send(&v[0], 10, MPI_INT, 1, 1, MPI_COMM_WORLD);
   }
   else
   {
     MPI_Recv(&v[0], 10, MPI_INT, MPI_ANY_SOURCE, 1, MPI_COMM_WORLD,&status);
     for(i=0;i<10;i++)
     {
        //change the value
        v[i] = i;
        //printf("hola soy nodo: %d\n", i);
     }
     MPI_Send(&v[0], 10, MPI_INT, 0, 2, MPI_COMM_WORLD);
   }

   if(rank==0)
   {
     MPI_Recv(&v[0], 10, MPI_INT, MPI_ANY_SOURCE, 2, MPI_COMM_WORLD,&status);
     for(i=1;i<size;i++)
     {
        printf("\nvalue of item %d: %d\n", i, v[i]);
     }
   }
}

void simpleFunction()
{
    printf("Hi i am a simple function\n");
}

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);
    MPI_Status status;

    int vect[10];
    int op=1;

    while(op != 0)
    {
        //system("clear");
        printf("Welcome to example program!\n\n");
        printf("\t1.- Call Parallel Function\n");
        printf("\t2.- Call another Function\n");
        printf("\t0.- Exit\n\t");

        printf("\n\n\tEnter option:");
        scanf("%d", &op);
        switch(op)
        {
            case 1:
                    my_parallel_function(vect, size, rank);
                    printf("Parallel function called successfully\n\n");
                    break;
            case 2:
                    simpleFunction();
                    printf("Function called successfully\n\n");
                    break;
        }
    }

    MPI_Finalize();
    return 0;
}

I am executing with -np 2

I think this is not woking as it should: MPI_Recv(&v[0], 10, MPI_INT, MPI_ANY_SOURCE, 2, MPI_COMM_WORLD,&status);

Any idea? thank you very much.

EDITED: But when i remove the if(rank==0) i get all the i/o of the slave processes, that means tons of times menu printing. I am novice in OpenMPI, how can solve this problem?.

Thank you very much!.

Answer 1

The proper way to implement a menu in an MPI program is:

while (op != 0)
{
   if (rank == 0)
   {
      printf("Welcome to example program!\n\n");
      printf("\t1.- Call Parallel Function\n");
      printf("\t2.- Call another Function\n");
      printf("\t0.- Exit\n\t");

      printf("\n\n\tEnter option:");
      scanf("%d", &op);
   }

   // Broadcast the user's choice to all other ranks
   MPI_Bcast(&op, MPI_INT, 1, 0, MPI_COMM_WORLD);

   switch(op)
   {
      case 1:
         my_parallel_function(vect, size, rank);
         MPI_Barrier(MPI_COMM_WORLD);
         if (rank == 0)
            printf("Parallel function called successfully\n\n");
         break;
      case 2:
         simpleFunction();
         MPI_Barrier(MPI_COMM_WORLD);
         if (rank == 0)
            printf("Function called successfully\n\n");
         break;
   }
}

The two barriers are not absolutely necessary - they just make sure that the calls have returned in all ranks.

Note that allowing rank 0 to read from the standard input is not guaranteed by the MPI standard and programs that do such kind of I/O are not portable. That said, most existing MPI implementations does indeed redirect the standard input and output of rank 0 to the standard input and output of mpiexec/mpirun.

Answer 2

my_parallel_function is only called when rank is 0. Therefore when rank is 1, nothing is sent to rank 0 and it's MPI_Recv waits forever. Just remove the if from your main.