[OMPI users] How is hwloc used by OpenMPI

[Blosch, Edwin L]

I see hwloc is a subproject hosted under OpenMPI but, in reading the 
documentation, I was unable to figure out if hwloc is a module within OpenMPI, 
or if some of the code base is borrowed into OpenMPI, or something else.  Is 
hwloc used by OpenMPI internally?  Is it a layer above libnuma?  Or is it just 
a project that is useful to OpenMPI in support of targeting various new 
platforms?

Thanks