ParaStation MPI is able to queue task start request if the required resources are currently in use. This queuing facility is disabled by default. It can be enabled by each user independently. All requests of all users are held within one queue and managed on a first-come-first-serve strategy ^[1] .

If queuing is not enabled, start up requests, which cannot immediately be satisfied due to resource constrains, e.g. too few CPUs are currently available, will terminate giving the error message

  PSI: PSI_createPartition: Resource temporarily unavailable.

To enable the queuing facility, an environment variable called PSI_WAIT must be defined within the user's environment:

  # export PSI_WAIT=""

The actual value of PSI_WAIT is not considered, only the existence of this variable is checked.

For task startup and process distribution, more environment variables are taken into account, e.g. PSI_OVERBOOK. See also the section called “Placing processes onto nodes”, process_placement(7) and ps_environment(5) .