MPI virtual graph topology broadcast

Question

I have a following problem:

I would like to create a virtual topology based on tree graph for example:

   0
 /   \
1    5
| \  | 
2  4 3

Vertices' numbers are ranks of processes.

I managed to do that and i have a handle on my new communicator:

MPI_Comm graph_comm;
MPI_Graph_create(MPI_COMM_WORLD, nnodes, indexes, edges, 0, &graph_comm);

Now my question is: Is there a possibility to send a broadcast (MPI_Bcast) from each of parent nodes that has children to their children only (in this example process with rank 0 sends bcast to processes 1, 5; process with rank 1 sends bcast to processes 2, 4; process with rank 5 sends bcast to process 3)

Answer 1

It seems to be impossible and one has to create separate communicators for broadcasting. While both MPI_Graph_neighbors_count and MPI_Graph_neighbors should be enough to create new groups, one might wonder why do we need graph topologies in the first place if those groups can be created with exactly the same data as graph topology would be?

Answer 2

Yes, you must create groups in every process and then you can call MPI_Bcast on each group, where root is parent of node (in your example 0 is parent for 1 & 5, but you should remember that root rank is assigned to local communicator so 0 does not have to be 0 in local group, it depends how you create it). This can help: Group and Communicator Management Routines