Launching with PBS / Torque
===========================

Open MPI supports PBS, PBS Pro, Torque, and other related resource
managers.

Verify PBS/Torque support
-------------------------

The ``prte_info`` command can be used to determine whether or not an
installed Open MPI includes Torque/PBS Pro support:

.. code-block::

   shell$ prte_info | grep ras

If the Open MPI installation includes support for PBS/Torque, you
should see a line similar to that below. Note the MCA version
information varies depending on which version of Open MPI is
installed.

.. code-block::

       MCA ras: tm (MCA v2.1.0, API v2.0.0, Component v3.0.0)

.. note:: PRRTE is the software layer that provides run-time
   environment support to Open MPI.  Open MPI typically hides most
   PMIx and PRRTE details from the end user, but this is one place
   that Open MPI is unable to hide the fact that PRRTE provides this
   functionality, not Open MPI.  Hence, users need to use the
   ``prte_info`` command to check for PBS/Torque support (not
   ``ompi_info``).

Launching
---------

When properly configured, Open MPI obtains both the list of hosts and
how many processes to start on each host from Torque / PBS Pro
directly.  Hence, it is unnecessary to specify the ``--hostfile``,
``--host``, or ``-n`` options to ``mpirun``.  Open MPI will use
PBS/Torque-native mechanisms to launch and kill processes (``ssh`` is
not required).

For example:

.. code-block:: sh

   # Allocate a PBS job with 4 nodes
   shell$ qsub -I -lnodes=4

   # Now run an Open MPI job on all the nodes allocated by PBS/Torque
   shell$ mpirun mpi-hello-world

This will run the MPI processes on the nodes that were allocated by
PBS/Torque.  Or, if submitting a script:

.. code-block:: sh

   shell$ cat my_script.sh
   #!/bin/sh
   mpirun mpi-hello-world
   shell$ qsub -l nodes=4 my_script.sh

.. warning:: Do not modify ``$PBS_NODEFILE``!

   We've had reports from some sites that system administrators modify
   the ``$PBS_NODEFILE`` in each job according to local policies.
   This will currently cause Open MPI to behave in an unpredictable
   fashion.  As long as no new hosts are added to the hostfile, it
   *usually* means that Open MPI will incorrectly map processes to
   hosts, but in some cases it can cause Open MPI to fail to launch
   processes altogether.