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General Run-Time Support Notes
==============================
* The Open MPI installation must be in your ``PATH`` on all nodes (and
potentially ``LD_LIBRARY_PATH`` or ``DYLD_LIBRARY_PATH``, if
``libmpi``/``libshmem`` is a shared library), unless using the
``--prefix`` or ``--enable-mpirun-prefix-by-default`` functionality (see
below).
* Open MPI's run-time behavior can be customized via Modular Component
Architecture (MCA) parameters (see :ref:`this section
<label-running-setting-mca-param-values>`) for more information on
how to get/set MCA parameter values). Some MCA parameters can be
set in a way that renders Open MPI inoperable. In particular, some
parameters have required options that must be included:
* If specified, the ``btl`` parameter must include the ``self``
component, or Open MPI will not be able to deliver messages to the
same rank as the sender. For example: ``mpirun --mca btl tcp,self
...``
* If specified, the ``btl_tcp_if_exclude`` parameter must include the
loopback device (``lo`` on many Linux platforms), or Open MPI will
not be able to route MPI messages using the TCP BTL. For example:
``mpirun --mca btl_tcp_if_exclude lo,eth1 ...``
* Running on nodes with different endian and/or different datatype
sizes within a single parallel job is supported in this release.
However, Open MPI does not resize data when datatypes differ in size
(for example, sending a 4 byte ``MPI_DOUBLE`` and receiving an 8 byte
``MPI_DOUBLE`` will fail).
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