SOS relies on PMI-simple for process startup and rank exchange, which is provided by the standalone Hydra process manager (mpiexec.hydra). Hydra can be downloaded and built independently of any full MPI implementation:

tar -xf hydra-<version>.tar.gz 
cd hydra-<version>

./configure --prefix=<hydrainstdir> 
make -j install

This installs <hydrainstdir>/bin/mpiexec.hydra, which is used later to launch the SOS and OSU OpenSHMEM benchmarks with PMI-simple.