cat > mpd.hosts hydra:8 vulpecula:4 taurus:4 centaurus:4 coma:4 fornax:4 ^DHere the number after the colon gives the number of processors on each machine.
You will only need to modify mpd.hosts (or make a different file) if you want to control which machines your tasks run on.
mpirun -f mpd.hosts hostnameThis runs the command 'hostname' on each of the machines specified in the mpd.hosts file. You should see that one version of the command is run per CPU.
mpirun -f mpd.hosts my-mcmc-code
One thing to bear in mind with the Royal Society machines is that they are not a uniform cluster. hydra has 8 CPUs (cores), while taurus, pictor, vulpecula, coma and fornax have 4. The speeds of the machines are also not identical: pictor and hydra are slightly slower than everything else.
Also bear in mind use of the nice command (or system call) in your MPI code to make sure that you don't screw up other people's interactive response. Don't forget that taurus, vulpecula and pictor are all desktop machines. If you need to run long, CPU-intensive jobs, consider using the Starlink cluster instead.