Setting Up The Grid

void Setup_grid(GRID_INFO_T* grid /* out */) {

int old_rank;

int dimensions[2];

int wrap_around[2];

int coordinates[2];

int free_coords[2];

/* Set up Global Grid Information */

MPI_Comm_size(MPI_COMM_WORLD, &(grid->p));

MPI_Comm_rank(MPI_COMM_WORLD, &old_rank);

/* We assume p is a perfect square */

grid->q = (int) sqrt((double) grid->p);

dimensions[0] = dimensions[1] = grid->q;

/* We want a circular shift in second dimension. */

/* Don't care about first */

wrap_around[0] = wrap_around[1] = 1;

MPI_Cart_create(MPI_COMM_WORLD, 2, dimensions,

wrap_around, 1, &(grid->comm));

MPI_Comm_rank(grid->comm, &(grid->my_rank));

MPI_Cart_coords(grid->comm, grid->my_rank, 2,

coordinates);

grid->my_row = coordinates[0];

grid->my_col = coordinates[1];

/* Set up row communicators */

free_coords[0] = 0;

free_coords[1] = 1;

MPI_Cart_sub(grid->comm, free_coords,

&(grid->row_comm));

/* Set up column communicators */

free_coords[0] = 1;

free_coords[1] = 0;

MPI_Cart_sub(grid->comm, free_coords,

&(grid->col_comm));

} /* Setup_grid */

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