[mpich-discuss] Is there any optimization of collective calls (MPI_Allreduce) for 2^n ranks?

[Jeff Hammond]

See papers by Sameer Kumar and coworkers on Google Scholar for
details.  You need to look at the pamid device code in MPICH to see
the collectives that are actually used on BGQ, but that won't actually
help since the all the BGQ collective magic is usually inside of PAMI.
PAMI is open-source but not exactly easy to understand.

Unlike previous generation of Blue Gene, BGQ collective optimizations
are much more general and often appear for communicator sizes other
than 2^n.