I recently updated my set-up, and because I use a High-Performance cluster from my University (kudos to avakas) to run various simulations and analyses, I have MPI and Rmpi installed on my laptop in order to test my scripts before submitting them to the big cluster. So I installed openmpi from homebrew very easily:
brew update brew install open-mpi
But then I had extensive trouble installing the Rmpi package…
After extensive investigation (and google searches), I landed on this stackoverflow thread. It turns out the problem comes from MacOS Sierra, which is generating quite long temporary folder names (by default). The solution is quite simple, simply add the following line in the openmpi-mca-params.conf file (located in /usr/local/Cellar/open-mpi/2.1.1/etc/ if you installed openmpi from homebrew):
orte_tmpdir_base = /tmp
This avoids having to set TMPDIR every time you use MPI ! After that I was able to install Rmpi and to run it smoothly without further tuning, all from within Rstudio.