domainDecom1D.c

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

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

    int rank, size, i, provided;
    
    // number of cells (global)
    int nxc = 1024; // make sure nxc is divisible by size
    double L = 2*3.1415; // Length of the domain
    

    MPI_Init_thread(&argc, &argv, MPI_THREAD_SINGLE, &provided);

    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // number of nodes (local to the process): 0 and nxn_loc-1 are ghost cells 
    int nxn_loc = nxc/size + 3; // number of nodes is number cells + 1; we add also 2 ghost cells
    double L_loc = L/((double) size);
    double dx = L / ((double) nxc);
    
    // define out function
    double *f = calloc(nxn_loc, sizeof(double)); // allocate and fill with z
    double *dfdx = calloc(nxn_loc, sizeof(double)); // allocate and fill with z

    for (i=1; i<(nxn_loc-1); i++)
      f[i] = sin(L_loc*rank + (i-1) * dx);
    
    // need to communicate and fill ghost cells f[0] and f[nxn_loc-1]
    // communicate ghost cells
    
    MPI_Send(&f[1], 1, MPI_DOUBLE, (rank - 1 + size) % size, rank, MPI_COMM_WORLD);
    // printf("P%d sent A to P%d through %d\n", rank, (rank - 1 + size) % size, rank);
    MPI_Send(&f[nxn_loc - 2], 1, MPI_DOUBLE, (rank + 1) % size, rank + size, MPI_COMM_WORLD);
    // printf("P%d sent B to P%d through %d\n", rank, (rank + 1) % size, rank + size);

    // printf("P%d try to recv B from P%d through %d\n", rank, (rank - 1 + size) % size, (rank - 1 + size) % size + size);
    MPI_Recv(&f[0], 1, MPI_DOUBLE, (rank - 1 + size) % size, (rank - 1 + size) % size + size, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    // printf("P%d try to recv A from P%d through %d\n", rank, (rank + 1) % size, (rank + 1) % size);
    MPI_Recv(&f[nxn_loc - 1], 1, MPI_DOUBLE, (rank + 1) % size, (rank + 1) % size, MPI_COMM_WORLD, MPI_STATUS_IGNORE);


    // here we finish the calculations

    // calculate first order derivative using central difference
    // here we need to correct value of the ghost cells!
    for (i=1; i<(nxn_loc-1); i++)
      dfdx[i] = (f[i+1] - f[i-1])/(2*dx);

    
    // Print f values
    if (rank==1){ // print only rank 0 for convenience
        printf("My rank %d of %d\n", rank, size );
        printf("Here are my values for f including ghost cells\n");
        for (i = 1; i < nxn_loc - 2; i++)
	       printf("dsin(x)/dx: %f, cos(x): %f\n", dfdx[i], cos(L_loc * rank + (i - 1) * dx));
        printf("\n");
    }   

    MPI_Finalize();
}