Hello, I have a couple of questions: 1) is there a way to find out what other processes reside in the same node as my_rank from within MPICH? I found this function: MPIR_Comm_is_node_aware(comm_ptr) and if it's true I can get access (although not always?) to comm_ptr->node_comm. I wonder what are the situations when I won't be able to know what processes are on the same node ( i.e. comm_ptr->node_comm is null) and is there any other way to get the info then? 2) MPIR_Comm_is_node_aware() returns true if comm->hierarchy_kind == MPID_HIERARCHY_PARENT and apparently there are the following hierarchies: /* flat, parent, node, or node_roots */ Could someone please explain what are each of the hierarchy? regards, -- Tatiana V. Martsinkevich PhD student INRIA Saclay, University of Paris Sud XI