Adding an array using MPI in C language

Question

I am very new to MPI programing. In the following code, I am trying to add first 3 elements using process 1 and last 3 elements using process 2. Result should show Sum is 11 for the first three elements and Sum is 18 for the last three elements. I see the reason for my problem is that at rank 2 (process 2) it reads the arr[3] as the very first element of the array. I am really lost about why it doesn't read the 3rd element of the array correctly at rank 2

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

int main (int argc, char *argv[])
{
        MPI_Init(&argc, &argv);
        int world_rank;
        MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
        int world_size;
        MPI_Comm_size(MPI_COMM_WORLD, &world_size);
        int tag2 = 1;
        int tag1 = 2;       
        int arr[7] = { 6,2,3,9,4,5};

        printf ("\n--Current Rank: %d\n", world_rank);
        int index;
        int source = 0;
        int dest;
        if (world_rank == 0)
        {
            printf("* Rank 0 excecuting");
            index = 0;
            dest = 1;
            MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
            MPI_Send(&arr, 2, MPI_INT, dest, tag2, MPI_COMM_WORLD); 

            index = 3;
            dest = 2;
            MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
            MPI_Send(&arr, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD);
        }
        else 
        {
            int sum = 0;
            int i;
            MPI_Recv(&index, 1, MPI_INT, source, tag1, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            MPI_Recv(&arr[index], 20, MPI_INT, source, tag2, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("At Rank: %d index is: %d\n", world_rank, index);
            for(i = index; i<=index+2; i++)
            {   
                printf("i: %d and arr[i]: %d\n", i, arr[i]);
                sum = arr[i]+sum;
            }
            printf("\n Sum is: %d and arr[3]: %d\n", sum, arr[3]);
        }       

        MPI_Finalize();
}

Result I get after using the command: mpirun -np 3 test1

--Current Rank: 0

--Current Rank: 1
At Rank: 1 index is: 0
i: 0 and arr[i]: 6
i: 1 and arr[i]: 2
i: 2 and arr[i]: 3

 Sum is: 11 and arr[3]: 9 //here it shows the correct value of 3rd array element

--Current Rank: 2
At Rank: 2 index is: 3
i: 3 and arr[i]: 6 //Error happens here
i: 4 and arr[i]: 4
i: 5 and arr[i]: 5

 Sum is: 15 and arr[3]: 6 //Show the wrong array value for 3rd element
* Rank 0 excecuting

Answer 1

Hum, the code does what you told it to do...

        index = 3;
        dest = 2;
        MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
        MPI_Send(&arr, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD);

Maybe you wanted more like:

        index = 3;
        dest = 2;
        MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
        MPI_Send(&arr[index], 3, MPI_INT, dest, tag2, MPI_COMM_WORLD);

And the fact that the other indexes were "valid" was because of the initial assignment...

There are also some more problems in this code:

1. int arr[7] = { 6,2,3,9,4,5};: maybe rather int arr[] = {6,2,3,9,4,5};

2. MPI_Send(&arr, 2, MPI_INT, dest, tag2, MPI_COMM_WORLD); should probably be MPI_Send(&arr[index], 3, MPI_INT, dest, tag2, MPI_COMM_WORLD); although I'm not too sure that is what you intended to do.

3. MPI_Recv(&arr[index], 20, MPI_INT, source, tag2, MPI_COMM_WORLD, MPI_STATUS_IGNORE);: Where does this 20 come from? Shouldn't it be 3 rather?

Hope this makes sense to you.

Answer 2

I am new to MPI. But this is what I know about MPI. Correct me if I am wrong.

• Usually process with rank 0 receives messages from other processes and prints the results or it will do the necessary calculations(in this case add the two values that it receives)

• All other processes do their part of work and sends it to process 0.

• Tags are unnecessary in this case because this only involves simple calculations.

So the solution for this can be simplified like this.

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

int main (int argc, char *argv[]){

        int world_rank,world_size, i;
        MPI_Init(&argc, &argv);
        MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
        MPI_Comm_size(MPI_COMM_WORLD, &world_size);  

        int arr[6] = {6,2,3,9,4,5};

        printf ("Current Rank: %d\n", world_rank);

        int total = 0;

        if(world_rank != 0){//if the rank is not 0, calculate the partial sum
            int tempTotal = 0;

            for(i = 0; i < 3; i++){
                tempTotal += arr[(world_rank -1)* 3 + i];
            }

            printf("* Rank %d sending value %d\n",world_rank, tempTotal);
            MPI_Send(&tempTotal, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);//send the partial sum to process 0
            printf("* Rank %d value sent\n", world_rank);

        }else{
            printf("RANK 0 receiving values:\n");

            int temp;

            for(i = 1; i < world_size; i++){
                MPI_Recv(&temp, 1, MPI_INT, i, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);//receive partial sums from other processes

                printf("Received value %d from rank %d\n", temp, i);

                total += temp;//add the partial sum to total
            }

            printf("Sum of the array = %d\n", total);//after all partial sums arrive, print the total
        }       
        MPI_Finalize();
}

To run the code:

mpicc mpi.c -o mpi
mpirun -np 3 ./mpi

Hope this helps.