Specify MPI collective algorithm

https://www.open-mpi.org/faq/?category=tuning#setting-mca-params

ompi_info --param coll tuned -l 9


          MCA coll tuned: parameter "coll_tuned_allreduce_algorithm" (current
                          value: "ignore", data source: default, level: 5
                          tuner/detail, type: int)
                          Which allreduce algorithm is used. Can be locked
                          down to any of: 0 ignore, 1 basic linear, 2
                          nonoverlapping (tuned reduce + tuned bcast), 3
                          recursive doubling, 4 ring, 5 segmented ring
                          Valid values: 0:"ignore", 1:"basic_linear",
                          2:"nonoverlapping", 3:"recursive_doubling",
                          4:"ring", 5:"segmented_ring", 6:"rabenseifner"

Specify additional flags to MPI:

mpi --mca coll_tuned_use_dynamic_rules 1 --mca coll_tuned_allreduce_algorithm 6