How to call same rank in MPI using C language?

Question

I am trying to learn MPI programing and have written the following program. It adds an entire row of array and outputs the sum. At rank 0 (or process 0), it will call all it

Answer 1

In MPI, each process executes the same code and, as you're doing, you differentiate the different processes primarily through checking rank in if/else statements. The master process with rank 0 is doing 3 sends: a send to process 1, then two sends to process 2. The slave processes each do only one receive, which means that rank 1 receives its first message and rank 2 receives its first message. When you call the third MPI_Send on process 0, there is not and will not be any slave waiting to receive the message after that point, as the slaves have finished executing their else block. The program gets blocked as the master waits to send the final message.

In order to fix that, you have to make sure the slave of rank 2 performs two receives, either by adding a loop for that process only or by repeating for that process only (so, using an if(world_rank == 2) check) the block of code

        sum = 0; //resetting sum
        MPI_Irecv(&arr[0], 1024, MPI_INT, source, tag2, MPI_COMM_WORLD, &request);
        MPI_Wait (&request, &status);
        for(i = 0; i<1024; i++)
        {   
            sum = arr[i]+sum;
        }
        printf("\nSum is: %d at rank: %d\n", sum, world_rank);