Care must be taken if the hardware is able to simulate virtual CPUs, e.g. Intel Xeon CPUs using Hyperthreading. The ParaStation MPI daemon detects virtual CPUs and uses all the virtual CPUs found for placing processes. Detecting virtual CPUs requires that the kernel module cpuid is loaded prior to starting the ParaStation MPI daemon. Use

  # psiadmin -c "s hw"
  Node     CPUs    Available Hardware
     0      4/ 2   ethernet p4sock
     1      4/ 2   ethernet p4sock

to show the number of virtual and physical CPUs per node.

It's possible to spawn more processes than physical or virtual CPUs are available on a node ("overbooking"). See ParaStation MPI User's Guide for details.