Cisco’s Jeff Squyres writes that Open MPI’s revamped v1.7 affinity system is actually quite flexible.
Open MPI v1.7 uses hwloc to probe the topologies of the nodes on which it is launching processes. These server topologies are gathered (and intelligently de-duplicated) and used to feed the mapping algorithm. Hence, not only do you not need to specify compute node topologies ahead of time (e.g., in a config file), the mapping will be performed correctly even if you are launching an MPI job across a heterogeneous set of servers.
For those looking for more fine-grained control, Squyres also provides some examples of how use the Expert Mode in v1.7. Read the Full Story.