I actually use MPI on a Raspberry Pi cluster, using the clusterHAT with 4 Raspberry Pi Zeros controlled by a Pi 3. The whole unit is about the size of a Big Mac

I actually use MPI on a Raspberry Pi cluster, using the clusterHAT with 4 Raspberry Pi Zeros controlled by a Pi 3. The whole unit is about the size of a Big Mac. Obviously it's not very performant, but the lessons learned on how to program with MPI in mind is where the value lies.