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Launching only on the local node
================================
It is common to develop MPI applications on a single workstation or
laptop, and then move to a larger parallel / HPC environment once the
MPI application is ready.
Open MPI supports running multi-process MPI jobs on a single machine.
In such cases, you can simply avoid listing a hostfile or remote
hosts, and simply list a number of MPI processes to launch. For
example:
.. code-block:: sh
shell$ mpirun -n 6 mpi-hello-world
Hello world, I am 0 of 6 (running on my-laptop))
Hello world, I am 1 of 6 (running on my-laptop)
...
Hello world, I am 5 of 6 (running on my-laptop)
If you do not specify the ``-n`` option, ``mpirun`` will default to
launching as many MPI processes as there are processor cores (not
hyperthreads) on the machine.
MPI communication
-----------------
When running on a single machine, Open MPI will most likely use the
``ob1`` PML and the following BTLs for MPI communication between
peers:
* ``self``: used for sending and receiving loopback MPI messages
|mdash| where the source and destination MPI process are the same.
* ``sm``: used for sending and receiving MPI messages where the source
and destination MPI processes can share memory (e.g., via SYSV or
POSIX shared memory mechanisms).
.. note:: For more information about using shared memory MPI
communication, see the :doc:`Shared Memory
</tuning-apps/networking/shared-memory>` page.
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