Splitting of MPI communicators

有时候为了创建2D的拓扑，需要对MPI_COMM_WORLD进行划分。所使用的MPI函数是：

 MPI_Comm_split(MPI_Comm comm, int color, int key, MPI_Comm *newcomm)

例子程序（出自MPI Groups, Communicators and Topologies）：

 #include
 #include

 main (int argc, char **argv)
 {
     MPI_Comm row_comm, col_comm;
     int myrank, size, P=4, Q=3, p, q;

     MPI_Init(&argc, &argv);
     MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
     MPI_Comm_size(MPI_COMM_WORLD, &size);

     p = myrank / Q;
     q = myrank % Q;

     /* Split comm intor row and column comms */
     MPI_Comm_split(MPI_COMM_WORLD, p, q, &row_comm);
     MPI_Comm_split(MPI_COMM_WORLD, q, p, &col_comm);

     int sum, rowsum, colsum;
     sum = myrank;
     rowsum = myrank;
     colsum = myrank;
     MPI_Allreduce(MPI_IN_PLACE, &sum, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
     MPI_Allreduce(MPI_IN_PLACE, &rowsum, 1, MPI_INT, MPI_SUM, row_comm);
     MPI_Allreduce(MPI_IN_PLACE, &colsum, 1, MPI_INT, MPI_SUM, col_comm);
     if (myrank == 0)
         printf("The sum of all rank is %d\n", sum);

     printf("[%d]: My coordinates are (%d, %d), row sum is %d, col sum is %d\n",myrank,p,q,
 rowsum,colsum);
     MPI_Finalize();
 }