Question

Write a program in  C  that does Old-Even sort  with MPI (i.e., cluster computing)

Answer

#include
#include
#include
#include

#define N 400

void swap(int* x, int* y)
{
    int swp = *x;
    *x = *y;
    *y = swp;
}

void print_array(int *array, int n)
{
    // print array's content 10 elements per row
    for (int i = 0; i < n; i++)
    {
        if (i % 10 == 0)
            printf("\n");
        printf("%d, ", array[i]);
    }
    printf("\n");
}

// serial selection sort
void sort(int *array, int n)
{
    for (int i = 0; i < n - 1; i++)
    {
        int k = i;
        for (int j = i + 1; j < n; j++)
            if (array[k] > array[j])
                k = j;
        
        swap(&array[i], &array[k]);
    }
}

int main(int argc, char **argv)
{
    // Initialize the MPI environment
    MPI_Init(&argc, &argv);
    
    // Get the number of processes
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    // Get the rank of the process
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    
    // a pointer to the array to be sorted
    int *array = NULL;
    
    if (world_rank == 0) // root process
    {
        srand(42);  // seed random number generator
        array = malloc(sizeof(int) * N);
        
        // populate array with random numbers in -100 .. 100 range
        for (int i = 0; i < N; i++)
        {
            array[i] = rand() % 200 - 100;
        }
        
        printf("\n\nBefore sorting: ");
        print_array(array, N);
    
    }
    // the number of array elements per process
    int chunk_size = N / world_size;
    // allocate memory for 2 arrays: one to hold a local 
    // copy of the array chunk and 
    // another for a neighbor's local copy
    int *local_array = malloc(sizeof(int) * chunk_size);
    int *other_array = malloc(sizeof(int) * chunk_size);
    
    // distribute array chunks between processes
    MPI_Scatter(array, chunk_size, MPI_INT, local_array, chunk_size, MPI_INT, 0, MPI_COMM_WORLD);
    
    for (int k = 0; k < world_size; k++)
    {
        // sort local array
        sort(local_array, chunk_size);
        
        // find the neighbor process for current (k-th) phase
        int other_rank = (k % 2 == world_rank % 2) ? world_rank + 1 : world_rank - 1;
        // skipping invalid neighbor rank
        if (other_rank < 0 || other_rank >= world_size)
            continue;
            
        // exchange local array data with neighbor (even process sends first)
        if (world_rank % 2 == 0) 
        {
            MPI_Send(local_array, chunk_size, MPI_INT, other_rank, 0, MPI_COMM_WORLD);
            MPI_Recv(other_array, chunk_size, MPI_INT, other_rank, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        } 
        else 
        {
            MPI_Recv(other_array, chunk_size, MPI_INT, other_rank, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            MPI_Send(local_array, chunk_size, MPI_INT, other_rank, 0, MPI_COMM_WORLD);
        }
        
        // order local and other arrays based on the relative order of their processes' ranks
        int *smaller_array = world_rank < other_rank ? local_array : other_array;
        int *larger_array = world_rank < other_rank ? other_array : local_array;
        
        // keep smaller elements in smaller_array and
        // larger elements in larger_array
        // larger array is iterated from the beginning
        // and smaller array is iterated from the end
        for (int i = 0; i < chunk_size; i++)
        {
            int j = chunk_size - 1 - i;
            if (smaller_array[j] <= larger_array[i])
                break;
            swap(&smaller_array[j], &larger_array[i]);
        }
        
    }
    
    // send ordered and sorted local_arrays to the root process
    MPI_Gather(local_array, chunk_size, MPI_INT, array, chunk_size, MPI_INT, 0, MPI_COMM_WORLD); 
    
    
    // deallocate memory
    free(other_array);
    free(local_array);  
    
    if (world_rank == 0)
    {
        printf("\n\nAfter sorting: ");
        print_array(array, N);
        
        free(array);  // deallocate memory
    }
    
    // Finalize the MPI environment.
    MPI_Finalize();
    
    return 0;
}