.. _man1-mpirun:
.. _man1-mpiexec:


mpirun / mpiexec
================

.. include_body

mpirun, mpiexec |mdash| Execute serial and parallel jobs in Open MPI.

.. note:: ``mpirun`` and ``mpiexec`` are synonyms for each other.
          Indeed, they are symbolic links to the same executable.
          Using either of the names will produce the exact same
          behavior.

SYNOPSIS
--------

Single Process Multiple Data (SPMD) Model:

.. code::

   mpirun [ options ] <program> [ <args> ]

Multiple Instruction Multiple Data (MIMD) Model:

.. code::

   mpirun [ global_options ]
          [ local_options1 ] <program1> [ <args1> ] :
          [ local_options2 ] <program2> [ <args2> ] :
          ... :
          [ local_optionsN ] <programN> [ <argsN> ]

Note that in both models, invoking ``mpirun`` via an absolute path
name is equivalent to specifying the ``--prefix`` option with a
``<dir>`` value equivalent to the directory where ``mpirun`` resides,
minus its last subdirectory.  For example:

.. code:: sh

   shell$ /usr/local/bin/mpirun ...

is equivalent to

.. code:: sh

   shell$ mpirun --prefix /usr/local

QUICK SUMMARY
-------------

If you are simply looking for how to run an MPI application, you
probably want to use a command line of the following form:

.. code:: sh

   shell$ mpirun [ -n X ] [ --hostfile <filename> ]  <program>

This will run X copies of <program> in your current run-time
environment (if running under a supported resource manager, Open MPI's
mpirun will usually automatically use the corresponding resource
manager process starter, as opposed to, for example, rsh or ssh, which
require the use of a hostfile, or will default to running all X copies
on the localhost), scheduling (by default) in a round-robin fashion by
CPU slot.  See the rest of this page for more details.

Please note that mpirun automatically binds processes as of the start
of the v1.8 series. Three binding patterns are used in the absence of
any further directives:

* Bind to core:     when the number of processes is <= 2
* Bind to socket:   when the number of processes is > 2
* Bind to none:     when oversubscribed

If your application uses threads, then you probably want to ensure
that you are either not bound at all (by specifying --bind-to none),
or bound to multiple cores using an appropriate binding level or
specific number of processing elements per application process.

.. _man1-mpirun-definition-of-slot:

DEFINITION OF 'SLOT'
--------------------

The term "slot" is used extensively in the rest of this manual page.
A slot is an allocation unit for a process.  The number of slots on a
node indicate how many processes can potentially execute on that node.
By default, Open MPI will allow one process per slot.

If Open MPI is not explicitly told how many slots are available on a
node (e.g., if a hostfile is used and the number of slots is not
specified for a given node), it will determine a maximum number of
slots for that node in one of two ways:

#. Default behavior: By default, Open MPI will attempt to discover the
   number of processor cores on the node, and use that as the number
   of slots available.

#. When ``--use-hwthread-cpus`` is used: If ``--use-hwthread-cpus`` is
   specified on the ``mpirun`` command line, then Open MPI will attempt to
   discover the number of hardware threads on the node, and use that
   as the number of slots available.

This default behavior also occurs when specifying the ``--host``
option with a single host.  Thus, the command:

.. code:: sh

   shell$ mpirun --host node1 ./a.out

launches a number of processes equal to the number of cores on node
``node1``, whereas:

.. code:: sh

   shell$ mpirun --host node1 --use-hwthread-cpus ./a.out

launches a number of processes equal to the number of hardware
threads on ``node1``.

When Open MPI applications are invoked in an environment managed by a
resource manager (e.g., inside of a SLURM job), and Open MPI was built
with appropriate support for that resource manager, then Open MPI will
be informed of the number of slots for each node by the resource
manager.  For example:

.. code:: sh

   shell$ mpirun ./a.out

launches one process for every slot (on every node) as dictated by
the resource manager job specification.

Also note that the one-process-per-slot restriction can be overridden
in unmanaged environments (e.g., when using hostfiles without a
resource manager) if oversubscription is enabled (by default, it is
disabled).  Most MPI applications and HPC environments do not
oversubscribe; for simplicity, the majority of this documentation
assumes that oversubscription is not enabled.

Slots are not hardware resources
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Slots are frequently incorrectly conflated with hardware resources.
It is important to realize that slots are an entirely different metric
than the number (and type) of hardware resources available.

Here are some examples that may help illustrate the difference:

#. More processor cores than slots: Consider a resource manager job
   environment that tells Open MPI that there is a single node with 20
   processor cores and 2 slots available.  By default, Open MPI will
   only let you run up to 2 processes.

   Meaning: you run out of slots long before you run out of processor
   cores.

#. More slots than processor cores: Consider a hostfile with a single
   node listed with a ``slots=50`` qualification.  The node has 20
   processor cores.  By default, Open MPI will let you run up to 50
   processes.

   Meaning: you can run many more processes than you have processor
   cores.

.. _man1-mpirun-definition-of-processor-element:

DEFINITION OF 'PROCESSOR ELEMENT'
---------------------------------

By default, Open MPI defines that a "processing element" is a
processor core.  However, if ``--use-hwthread-cpus`` is specified on the
mpirun command line, then a "processing element" is a hardware thread.

OPTIONS
-------

mpirun will send the name of the directory where it was invoked on the
local node to each of the remote nodes, and attempt to change to that
directory.  See the "Current Working Directory" section below for
further details.

* ``<program>``: The program executable. This is identified as the
  first non-recognized argument to mpirun.

* ``<args>``: Pass these run-time arguments to every new process.
  These must always be the last arguments to mpirun. If an app context
  file is used, ``<args>`` will be ignored.

* ``-h``, ``--help``: Display help for this command

* ``-q``, ``--quiet``: Suppress informative messages from orterun
  during application execution.

* ``-v``, ``--verbose``:` Be verbose

* ``-V``, ``--version``: Print version number.  If no other arguments
  are given, this will also cause orterun to exit.

* ``-N <num>``: Launch num processes per node on all allocated nodes
  (synonym for ``--npernode``).

* ``--display-map``: Display a table showing the mapped location of
  each process prior to launch.

* ``--display-allocation``: Display the detected resource allocation.

* ``--output-proctable``: Output the debugger proctable after launch.

* ``--dvm``: Create a persistent distributed virtual machine (DVM).

* ``--max-vm-size <size>``: Number of processes to run.

* ``--novm``: Execute without creating an allocation-spanning virtual
  machine (only start daemons on nodes hosting application procs).

* ``--hnp <arg0>``: Specify the URI of the Head Node Process (HNP), or
  the name of the file (specified as ``file:filename``) that contains
  that info.

Use one of the following options to specify which hosts (nodes) of the
cluster to run on. Note that as of the start of the v1.8 release,
mpirun will launch a daemon onto each host in the allocation (as
modified by the following options) at the very beginning of execution,
regardless of whether or not application processes will eventually be
mapped to execute there. This is done to allow collection of hardware
topology information from the remote nodes, thus allowing us to map
processes against known topology. However, it is a change from the
behavior in prior releases where daemons were only launched after
mapping was complete, and thus only occurred on nodes where
application processes would actually be executing.

* ``-H``, ``--host <host1,host2,...,hostN>``: list of hosts on which to
  invoke processes.

* ``--hostfile <hostfile>``: Provide a hostfile to use.

* ``--default-hostfile <hostfile>``: Provide a default hostfile.

* ``--machinefile <machinefile>``: Synonym for ``--hostfile``.

* ``--cpu-set <list>``: Restrict launched processes to the specified
  logical CPUs on each node (comma-separated list). Note that the
  binding options will still apply within the specified envelope
  |mdash| e.g., you can elect to bind each process to only one CPU
  within the specified CPU set.

The following options specify the number of processes to launch. Note
that none of the options imply a particular binding policy |mdash| e.g.,
requesting N processes for each socket does not imply that the
processes will be bound to the socket.

* ``-n``, ``--n``, ``-c``, ``-np <#>``: Run this many copies of the
  program on the given nodes.  This option indicates that the
  specified file is an executable program and not an application
  context. If no value is provided for the number of copies to execute
  (i.e., neither the ``-n`` nor its synonyms are provided on the
  command line), Open MPI will automatically execute a copy of the
  program on each process slot (see below for description of a
  "process slot"). This feature, however, can only be used in the SPMD
  model and will return an error (without beginning execution of the
  application) otherwise.

  .. note:: The ``-n`` option is the preferred option to be used to specify the
            number of copies of the program to be executed, but the alternate
            options are also accepted.


* ``--map-by ppr:N:<object>``: Launch N times the number of objects of
  the specified type on each node.

* ``--npersocket <#persocket>``: On each node, launch this many
  processes times the number of processor sockets on the node.
  The -npersocket option also turns on the ``--bind-to-socket``
  option.  (deprecated in favor of ``--map-by ppr:n:socket``)

* ``--npernode <#pernode>``: On each node, launch this many processes.
  (deprecated in favor of ``--map-by ppr:n:node``).

* ``--pernode``: On each node, launch one process |mdash| equivalent to
  ``--npernode 1``.  (deprecated in favor of ``--map-by ppr:1:node``)

To map processes:

* ``--map-by <object>``: Map to the specified object, defaults to
  socket. Supported options include ``slot``, ``hwthread``, ``core``,
  ``L1cache``, ``L2cache``, ``L3cache``, ``socket``, ``numa``,
  ``board``, ``node``, ``sequential``, ``distance``, and ``ppr``. Any
  object can include modifiers by adding a ``:`` and any combination
  of ``PE=n`` (bind n processing elements to each proc), ``SPAN``
  (load balance the processes across the allocation),
  ``OVERSUBSCRIBE`` (allow more processes on a node than processing
  elements), and ``NOOVERSUBSCRIBE``.  This includes ``PPR``, where
  the pattern would be terminated by another colon to separate it from
  the modifiers.

* ``--bycore``: Map processes by core (deprecated in favor of
  ``--map-by core``)

* ``--byslot``: Map and rank processes round-robin by slot.

* ``--nolocal``: Do not run any copies of the launched application on
  the same node as orterun is running.  This option will override
  listing the localhost with ``--host`` or any other host-specifying
  mechanism.

* ``--nooversubscribe``: Do not oversubscribe any nodes; error
  (without starting any processes) if the requested number of
  processes would cause oversubscription.  This option implicitly sets
  "max_slots" equal to the "slots" value for each node. (Enabled by
  default).

* ``--oversubscribe``: Nodes are allowed to be oversubscribed, even on
  a managed system, and overloading of processing elements.

* ``--bynode``: Launch processes one per node, cycling by node in a
  round-robin fashion.  This spreads processes evenly among nodes and
  assigns MPI_COMM_WORLD ranks in a round-robin, "by node" manner.

* ``--cpu-list <cpus>``: Comma-delimited list of processor IDs to
  which to bind processes [default=NULL].  Processor IDs are
  interpreted as hwloc logical core IDs.

  .. note:: You can run Run the hwloc ``lstopo(1)`` command to see a
            list of available cores and their logical IDs.

To order processes' ranks in MPI_COMM_WORLD:

* ``--rank-by <object>``: Rank in round-robin fashion according to the
  specified object, defaults to slot. Supported options include
  ``slot``, ``hwthread``, ``core``, ``L1cache``, ``L2cache``,
  ``L3cache``, ``socket``, ``numa``, ``board``, and ``node``.

For process binding:

* ``--bind-to <object>``: Bind processes to the specified object,
  defaults to ``core``.  Supported options include ``slot``,
  ``hwthread``, ``core``, ``l1cache``, ``l2cache``, ``l3cache``,
  ``socket``, ``numa``, ``board``, ``cpu-list``, and ``none``.

* ``--cpus-per-proc <#perproc>``: Bind each process to the specified
  number of cpus.  (deprecated in favor of ``--map-by <obj>:PE=n``)

* ``--cpus-per-rank <#perrank>``: Alias for ``--cpus-per-proc``.
  (deprecated in favor of ``--map-by <obj>:PE=n``)

* ``--bind-to-core`` Bind processes to cores (deprecated in favor of
  ``--bind-to core``)

* ``--bind-to-socket``: Bind processes to processor sockets
  (deprecated in favor of ``--bind-to socket``)

* ``--report-bindings``: Report any bindings for launched processes.

For rankfiles:

* ``--rankfile <rankfile>``: Provide a rankfile file.

To manage standard I/O:

* ``--output-filename <filename>``: Redirect the stdout, stderr, and
  stddiag of all processes to a process-unique version of the
  specified filename. Any directories in the filename will
  automatically be created.  Each output file will consist of
  ``filename.id``, where the ``id`` will be the processes' rank in
  MPI_COMM_WORLD, left-filled with zero's for correct ordering in
  listings. A relative path value will be converted to an absolute
  path based on the cwd where mpirun is executed. Note that this will
  not work on environments where the file system on compute nodes
  differs from that where :ref:`mpirun(1) <man1-mpirun>` is
  executed.

* ``--stdin <rank>``: The MPI_COMM_WORLD rank of the process that is
  to receive stdin.  The default is to forward stdin to MPI_COMM_WORLD
  rank 0, but this option can be used to forward stdin to any
  process. It is also acceptable to specify none, indicating that no
  processes are to receive stdin.

* ``--merge-stderr-to-stdout``: Merge stderr to stdout for each
  process.

* ``--tag-output``: Tag each line of output to stdout, stderr, and
  stddiag with ``[jobid, MCW_rank]<stdxxx>`` indicating the process
  jobid and MPI_COMM_WORLD rank of the process that generated the
  output, and the channel which generated it.

* ``--timestamp-output``: Timestamp each line of output to stdout,
  stderr, and stddiag.

* ``--xml``: Provide all output to stdout, stderr, and stddiag in an
  XML format.

* ``--xml-file <filename>`` Provide all output in XML format to the
  specified file.

* ``--xterm <ranks>``: Display the output from the processes
  identified by their MPI_COMM_WORLD ranks in separate xterm
  windows. The ranks are specified as a comma-separated list of
  ranges, with a -1 indicating all. A separate window will be created
  for each specified process.

  .. note:: xterm will normally terminate the window upon termination
            of the process running within it. However, by adding a
            ``!`` to the end of the list of specified ranks, the
            proper options will be provided to ensure that xterm keeps
            the window open after the process terminates, thus
            allowing you to see the process' output.  Each xterm
            window will subsequently need to be manually closed.
            Note: In some environments, xterm may require that the
            executable be in the user's path, or be specified in
            absolute or relative terms. Thus, it may be necessary to
            specify a local executable as ``./my_mpi_app`` instead of just
            ``my_mpi_app``. If xterm fails to find the executable, ``mpirun``
            will hang, but still respond correctly to a ctrl-C.  If
            this happens, please check that the executable is being
            specified correctly and try again.

To manage files and runtime environment:

* ``--path <path>``: ``<path>`` that will be used when attempting to
  locate the requested executables.  This is used prior to using the
  local ``PATH`` environment variable setting.

* ``--prefix <dir>``: Prefix directory that will be used to set the
  ``PATH`` and ``LD_LIBRARY_PATH`` on the remote node before invoking
  Open MPI or the target process.  See the :ref:`Remote Execution
  <man1-mpirun-remote-execution>` section, below.

* ``--noprefix``: Disable the automatic ``--prefix`` behavior

* ``--preload-binary``: Copy the specified executable(s) to remote
  machines prior to starting remote processes. The executables will be
  copied to the Open MPI session directory and will be deleted upon
  completion of the job.

* ``--preload-files <files>``: Preload the comma-separated list of
  files to the current working directory of the remote machines where
  processes will be launched prior to starting those processes.

* ``--set-cwd-to-session-dir``: Set the working directory of the
  started processes to their session directory.

* ``--wd <dir>``: Synonym for ``-wdir``.

* ``--wdir <dir>``: Change to the directory ``<dir>`` before the
  user's program executes.  See the :ref:`Current Working Directory
  <man1-mpirun-current-working-directory>` section for notes on
  relative paths.  Note: If the ``--wdir`` option appears both on the
  command line and in an application context, the context will take
  precedence over the command line. Thus, if the path to the desired
  wdir is different on the backend nodes, then it must be specified as
  an absolute path that is correct for the backend node.

* ``-x <env>``: Export the specified environment variables to the
  remote nodes before executing the program.  Only one environment
  variable can be specified per ``-x`` option.  Existing environment
  variables can be specified or new variable names specified with
  corresponding values.  For example:

  .. code:: sh

     shell$ mpirun -x DISPLAY -x OFILE=/tmp/out ...

  The parser for the ``-x`` option is not very sophisticated; it does
  not even understand quoted values.  Users are advised to set
  variables in the environment, and then use ``-x`` to export (not
  define) them.

Setting MCA parameters:

* ``--gmca <key> <value>``: Pass global MCA parameters that are
  applicable to all contexts.  ``<key>`` is the parameter name;
  ``<value>`` is the parameter value.

* ``--mca <key> <value>``: Send arguments to various MCA modules.  See
  the :ref:`Setting MCA Parameters
  <man1-mpirun-setting-mca-parameters>` section for mode details.

* ``--am <arg0>``: Aggregate MCA parameter set file list.

* ``--tune <tune_file>``: Specify a tune file to set arguments for
  various MCA modules and environment variables.  See the :ref:`
  Setting MCA parameters and environment variables from file
  <man1-mpirun-setting-mca-params-from-file>`

For debugging:

* ``--debug``: Invoke the user-level debugger indicated by the
  ``orte_base_user_debugger`` MCA parameter.

* ``--get-stack-traces``: When paired with the ``--timeout`` option,
  ``mpirun`` will obtain and print out stack traces from all launched
  processes that are still alive when the timeout expires.  Note that
  obtaining stack traces can take a little time and produce a lot of
  output, especially for large process-count jobs.

* ``--debugger <args>``: Sequence of debuggers to search for when
  ``--debug`` is used (i.e., a synonym for the
  ``orte_base_user_debugger`` MCA parameter).

* ``--timeout <seconds>``: The maximum number of seconds that
  ``mpirun`` will run.  After this many seconds, ``mpirun`` will abort
  the launched job and exit with a non-zero exit status.  Using
  ``--timeout`` can be also useful when combined with the
  ``--get-stack-traces`` option.

* ``--tv``: Launch processes under a debugger.  Deprecated backwards
  compatibility flag. Synonym for ``--debug``.

There are also other options:

* ``--allow-run-as-root``: Allow ``mpirun`` to run when executed by
  the root user (``mpirun`` defaults to aborting when launched as the
  root user).  Be sure to see the :ref:`Running as root
  <man1-mpirun-running-as-root>` section for more detail.

* ``--app <appfile>``: Provide an appfile, ignoring all other command
  line options.

* ``--cartofile <cartofile>``: Provide a cartography file.

* ``--continuous``: Job is to run until explicitly terminated.

* ``--disable-recovery``: Disable recovery (resets all recovery
  options to off).

* ``--do-not-launch``: Perform all necessary operations to prepare to
  launch the application, but do not actually launch it.

* ``--do-not-resolve``: Do not attempt to resolve interfaces.

* ``--enable-recovery``: Enable recovery from process failure (default:
  disabled)

* ``--index-argv-by-rank``: Uniquely index argv[0] for each process
  using its rank.

* ``--leave-session-attached``: Do not detach back-end daemons used by
  this application. This allows error messages from the daemons as
  well as the underlying environment (e.g., when failing to launch a
  daemon) to be output.

* ``--max-restarts <num>``: Max number of times to restart a failed
  process.

* ``--ompi-server <uri or file>``: Specify the URI of the Open MPI
  server (or the mpirun to be used as the server), the name of the
  file (specified as ``file:filename``) that contains that info, or
  the PID (specified as ``pid:#``) of the mpirun to be used as the
  server.  The Open MPI server is used to support multi-application
  data exchange via the :ref:`MPI_Publish_name(3) <mpi_publish_name>`
  and :ref:`MPI_Lookup_name(3) <mpi_lookup_name>` functions.

* ``--personality <list>``: Comma-separated list of programming model,
  languages, and containers being used (default=``ompi``).

* ``--ppr <list>``: Comma-separated list of number of processes on a
  given resource type (default: none).

* ``--report-child-jobs-separately``: Return the exit status of the
  primary job only.

* ``--report-events <URI>``: Report events to a tool listening at the
  specified URI.

* ``--report-pid <channel>``: Print out ``mpirun``'s PID during
  startup. The channel must be either a ``-`` to indicate that the PID
  is to be output to stdout, a ``+`` to indicate that the PID is to be
  output to stderr, or a filename to which the PID is to be written.

* ``--report-uri <channel>``: Print out ``mpirun``'s URI during
  startup. The channel must be either a ``-`` to indicate that the URI
  is to be output to stdout, a ``+`` to indicate that the URI is to be
  output to stderr, or a filename to which the URI is to be written.

* ``--show-progress``: Output a brief periodic report on launch
  progress.

* ``--terminate``: Terminate the DVM.

* ``--use-hwthread-cpus``: Use hardware threads as independent CPUs.

  Note that if a number of slots is not provided to Open MPI (e.g.,
  via the ``slots`` keyword in a hostfile or from a resource manager
  such as SLURM), the use of this option changes the default
  calculation of number of slots on a node.  See the :ref:`DEFINITION
  OF 'SLOT' <man1-mpirun-definition-of-slot>` section.

  Also note that the use of this option changes the Open MPI's
  definition of a "processor element" from a processor core to a
  hardware thread.  See the :ref:`DEFINITION OF 'PROCESSOR ELEMENT'
  <man1-mpirun-definition-of-processor-element>` section.

* ``--use-regexp``: Use regular expressions for launch.

The following options are useful for developers; they are not
generally useful to most Open MPI users:

* ``-d``, ``--debug-devel``: Enable debugging of the back-end run-time
  system.  This is not generally useful for most users.

* ``--debug-daemons``: Enable debugging of the run-time daemons used
  by this application.

* ``--debug-daemons-file``: Enable debugging of the run-time daemons
  used by this application, storing output in files.

* ``--display-devel-allocation``:
  Display a detailed list of the allocation being used by this job.

* ``--display-devel-map``: Display a more detailed table showing the
  mapped location of each process prior to launch.

* ``--display-diffable-map``: Display a diffable process map just
  before launch.

* ``--display-topo``: Display the topology as part of the process map
  just before launch.

* ``--launch-agent``: Name of the executable that is to be used to
  start processes on the remote nodes. The default is ``prted``. This
  option can be used to test new daemon concepts, or to pass options
  back to the daemons without having mpirun itself see them. For
  example, specifying a launch agent of ``prted -mca odls_base_verbose
  5`` allows the developer to ask the ``prted`` for debugging output
  without clutter from ``mpirun`` itself.

* ``--report-state-on-timeout``: When paired with the ``--timeout``
  command line option, report the run-time subsystem state of each
  process when the timeout expires.

There may be other options listed with ``mpirun --help``.

Environment Variables
^^^^^^^^^^^^^^^^^^^^^

* ``MPIEXEC_TIMEOUT``: Synonym for the ``--timeout`` command line option.

DESCRIPTION
-----------

One invocation of ``mpirun`` starts an MPI application running under Open
MPI. If the application is single process multiple data (SPMD), the
application can be specified on the ``mpirun`` command line.

If the application is multiple instruction multiple data (MIMD),
comprising of multiple programs, the set of programs and argument can
be specified in one of two ways: Extended Command Line Arguments, and
Application Context.

An application context describes the MIMD program set including all
arguments in a separate file.  This file essentially contains multiple
mpirun command lines, less the command name itself.  The ability to
specify different options for different instantiations of a program is
another reason to use an application context.

Extended command line arguments allow for the description of the
application layout on the command line using colons (``:``) to
separate the specification of programs and arguments. Some options are
globally set across all specified programs (e.g., ``--hostfile``),
while others are specific to a single program (e.g., ``-n``).

Specifying Host Nodes
^^^^^^^^^^^^^^^^^^^^^

Host nodes can be identified on the ``mpirun`` command line with the
``--host`` option or in a hostfile.

For example:

.. code:: sh

   shell$ mpirun -H aa,aa,bb ./a.out

Launches two processes on node ``aa`` and one on ``bb``.

Or, consider the hostfile:

.. code:: sh

   shell$ cat myhostfile
   aa slots=2
   bb slots=2
   cc slots=2

Here, we list both the host names (``aa``, ``bb``, and ``cc``) but
also how many slots there are for each.

.. code:: sh

   shell$ mpirun --hostfile myhostfile ./a.out

will launch two processes on each of the three nodes.

.. code:: sh

   shell$ mpirun --hostfile myhostfile --host aa ./a.out

will launch two processes, both on node ``aa``.

.. code:: sh

   shell$ mpirun --hostfile myhostfile --host dd ./a.out

will find no hosts to run on and will abort with an error.  That is,
the specified host ``dd`` is not in the specified hostfile.

When running under resource managers (e.g., SLURM, Torque, etc.), Open
MPI will obtain both the hostnames and the number of slots directly
from the resource manager.

Specifying Number of Processes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As we have just seen, the number of processes to run can be set using the
hostfile.  Other mechanisms exist.

The number of processes launched can be specified as a multiple of the
number of nodes or processor sockets available.  For example,

.. code:: sh

   shell$ mpirun -H aa,bb --npersocket 2 ./a.out

launches processes 0-3 on node ``aa`` and process 4-7 on node ``bb``
(assuming ``aa`` and ``bb`` both contain 4 slots each).  The
``--npersocket`` option also turns on the ``--bind-to-socket`` option,
which is discussed in a later section.

.. code:: sh

   shell$ mpirun -H aa,bb --npernode 2 ./a.out

launches processes 0-1 on node ``aa`` and processes 2-3 on node ``bb``.

.. code:: sh

   shell$ mpirun -H aa,bb --npernode 1 ./a.out

launches one process per host node.

.. code:: sh

   mpirun -H aa,bb --pernode ./a.out

is the same as ``--npernode 1``.

Another alternative is to specify the number of processes with the ``-n``
option.  Consider now the hostfile:

.. code:: sh

   shell$ cat myhostfile
   aa slots=4
   bb slots=4
   cc slots=4

Now run with ``myhostfile``:

.. code:: sh

   shell$ mpirun --hostfile myhostfile -n 6 ./a.out

will launch processes 0-3 on node ``aa`` and processes 4-5 on node
``bb``.  The remaining slots in the hostfile will not be used since
the ``-n`` option indicated that only 6 processes should be launched.

Mapping Processes to Nodes: Using Policies
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The examples above illustrate the default mapping of process processes
to nodes.  This mapping can also be controlled with various ``mpirun``
options that describe mapping policies.

Consider the same hostfile as above, again with ``-n 6``.  The table
below lists a few ``mpirun`` variations, and shows which
MPI_COMM_WORLD ranks end up on which node:

.. list-table::
   :header-rows: 1

   * - Command
     - Node ``aa``
     - Node ``bb``
     - Node ``cc``

   * - ``mpirun``
     - 0 1 2 3
     - 4 5
     -

   * - ``mpirun --map-by node``
     - 0 3
     - 1 4
     - 2 5

   * - ``mpirun --nolocal``
     -
     - 0 1 2 3
     - 4 5

The ``--map-by node`` option will load balance the processes across the
available nodes, numbering each process in a round-robin fashion.

The ``--nolocal`` option prevents any processes from being mapped onto
the local host (in this case node ``aa``).  While ``mpirun`` typically
consumes few system resources, ``--nolocal`` can be helpful for
launching very large jobs where mpirun may actually need to use
noticeable amounts of memory and/or processing time.

Just as ``-n`` can specify fewer processes than there are slots, it
can also oversubscribe the slots.  For example, with the same
hostfile:

.. code:: sh

   shell$ mpirun --hostfile myhostfile -n 14 ./a.out

will launch processes 0-3 on node ``aa``, 4-7 on ``bb``, and 8-11 on
``cc``.  It will then add the remaining two processes to whichever
nodes it chooses.

One can also specify limits to oversubscription.  For example, with the
same hostfile:

.. code:: sh

   shell$ mpirun --hostfile myhostfile -n 14 --nooversubscribe ./a.out

will produce an error since ``--nooversubscribe`` prevents
oversubscription.

Limits to oversubscription can also be specified in the hostfile
itself:

.. code:: sh

   shell$ cat myhostfile
   aa slots=4 max_slots=4
   bb         max_slots=4
   cc slots=4

The ``max_slots`` field specifies such a limit.  When it does, the slots
value defaults to the limit.  Now:

.. code:: sh

   shell$ mpirun --hostfile myhostfile -n 14 ./a.out

causes the first 12 processes to be launched as before, but the
remaining two processes will be forced onto node ``cc``.  The other
two nodes are protected by the hostfile against oversubscription by
this job.

Using the ``--nooversubscribe`` option can be helpful since Open MPI
currently does not get ``max_slots`` values from the resource manager.

Of course, ``-n`` can also be used with the ``-H`` or ``-host``
option.  For example:

.. code:: sh

   shell$ mpirun -H aa,bb -n 8 ./a.out

launches 8 processes.  Since only two hosts are specified, after the
first two processes are mapped, one to ``aa`` and one to ``bb``, the
remaining processes oversubscribe the specified hosts.

And here is a MIMD example:

.. code:: sh

   shell$ mpirun -H aa -n 1 hostname : -H bb,cc -n 2 uptime

will launch process 0 running hostname on node ``aa`` and processes 1
and 2 each running uptime on nodes ``bb`` and ``cc``, respectively.

Mapping, Ranking, and Binding: Oh My!
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Open MPI employs a three-phase procedure for assigning process locations
and ranks:

#. Mapping: Assigns a default location to each process
#. Ranking: Assigns an MPI_COMM_WORLD rank value to each process
#. Binding: Constrains each process to run on specific processors

The mapping step is used to assign a default location to each process
based on the mapper being employed. Mapping by slot, node, and
sequentially results in the assignment of the processes to the node
level. In contrast, mapping by object, allows the mapper to assign the
process to an actual object on each node.

Note that the location assigned to the process is independent of where
it will be bound |mdash| the assignment is used solely as input to the
binding algorithm.

The mapping of process processes to nodes can be defined not just with
general policies but also, if necessary, using arbitrary mappings that
cannot be described by a simple policy.  One can use the "sequential
mapper," which reads the hostfile line by line, assigning processes to
nodes in whatever order the hostfile specifies.  Use the ``--mca rmaps
seq`` option.  For example, using the same hostfile as before:

.. code:: sh

   shell$ mpirun -hostfile myhostfile -mca rmaps seq ./a.out

will launch three processes, one on each of nodes aa, bb, and cc,
respectively.  The slot counts don't matter; one process is launched
per line on whatever node is listed on the line.

Another way to specify arbitrary mappings is with a rankfile, which
gives you detailed control over process binding as well.  Rankfiles
are discussed below.

The second phase focuses on the ranking of the process within the
job's MPI_COMM_WORLD.  Open MPI separates this from the mapping
procedure to allow more flexibility in the relative placement of MPI
processes. This is best illustrated by considering the following two
cases where we used the ``map-by ppr:2:socket`` option:

.. list-table::
   :header-rows: 1

   * - Option
     - Node ``aa``
     - Node ``bb``

   * - ``--rank-by core``
     - 0 1 | 2 3
     - 4 5 | 6 7

   * - ``--rank-by socket``
     - 0 2 | 1 3
     - 4 6 | 5 7

   * - ``--rank-by socket:span``
     - 0 4 | 1 5
     - 2 6 | 3 7

Ranking by core and by slot provide the identical result |mdash| a
simple progression of MPI_COMM_WORLD ranks across each node. Ranking
by socket does a round-robin ranking within each node until all
processes have been assigned an MCW rank, and then progresses to the
next node. Adding the ``span`` modifier to the ranking directive causes
the ranking algorithm to treat the entire allocation as a single
entity |mdash| thus, the MCW ranks are assigned across all sockets
before circling back around to the beginning.

The binding phase actually binds each process to a given set of
processors. This can improve performance if the operating system is
placing processes suboptimally.  For example, it might oversubscribe
some multi-core processor sockets, leaving other sockets idle; this
can lead processes to contend unnecessarily for common resources.  Or,
it might spread processes out too widely; this can be suboptimal if
application performance is sensitive to interprocess communication
costs.  Binding can also keep the operating system from migrating
processes excessively, regardless of how optimally those processes
were placed to begin with.

The processors to be used for binding can be identified in terms of
topological groupings |mdash| e.g., binding to an ``l3cache`` will
bind each process to all processors within the scope of a single L3
cache within their assigned location. Thus, if a process is assigned
by the mapper to a certain socket, then a ``--bind-to l3cache``
directive will cause the process to be bound to the processors that
share a single L3 cache within that socket.

Alternatively, processes can be assigned to processors based on their
local rank on a node using the ``--bind-to cpu-list:ordered`` option with
an associated ``--cpu-list 0,2,5``. In this example, the first process
on a node will be bound to CPU 0, the second process on the node will
be bound to CPU 2, and the third process on the node will be bound to
CPU 5.

``--bind-to`` will also accept ``cpulist:ordered`` as a synonym to
``cpu-list:ordered``.  Note that an error will result if more
processes are assigned to a node than CPUs are provided.

To help balance loads, the binding directive uses a round-robin method
when binding to levels lower than used in the mapper. For example,
consider the case where a job is mapped to the socket level, and then
bound to core. Each socket will have multiple cores, so if multiple
processes are mapped to a given socket, the binding algorithm will
assign each process located to a socket to a unique core in a
round-robin manner.

Alternatively, processes mapped by ``l2cache`` and then bound to socket
will simply be bound to all the processors in the socket where they
are located. In this manner, users can exert detailed control over
relative MCW rank location and binding.

Finally, ``--report-bindings`` can be used to report bindings.

As an example, consider a node with two processor sockets, each
comprised of four cores, and each of those cores contains one hardware
thread.  We run mpirun with ``-n 4 --report-bindings`` and the
following additional options:

.. code::

   shell$ mpirun ... --map-by core --bind-to core
   [...] ... binding child [...,0] to cpus 0001
   [...] ... binding child [...,1] to cpus 0002
   [...] ... binding child [...,2] to cpus 0004
   [...] ... binding child [...,3] to cpus 0008

   shell$ mpirun ... --map-by socket --bind-to socket
   [...] ... binding child [...,0] to socket 0 cpus 000f
   [...] ... binding child [...,1] to socket 1 cpus 00f0
   [...] ... binding child [...,2] to socket 0 cpus 000f
   [...] ... binding child [...,3] to socket 1 cpus 00f0

   shell$ mpirun ... --map-by slot:PE=2 --bind-to core
   [...] ... binding child [...,0] to cpus 0003
   [...] ... binding child [...,1] to cpus 000c
   [...] ... binding child [...,2] to cpus 0030
   [...] ... binding child [...,3] to cpus 00c0

   shell$ mpirun ... --bind-to none

.. error:: TODO Is this still right?  Don't we show something more
           user-friendly these days?

Here, ``--report-bindings`` shows the binding of each process as a
mask.  In the first case, the processes bind to successive cores as
indicated by the masks 0001, 0002, 0004, and 0008.  In the second
case, processes bind to all cores on successive sockets as indicated
by the masks 000f and 00f0.  The processes cycle through the processor
sockets in a round-robin fashion as many times as are needed.

In the third case, the masks show us that 2 cores have been bound per
process.  Specifically, the mapping by slot with the PE=2 qualifier
indicated that each slot (i.e., process) should consume two processor
elements.  Since ``--use-hwthread-cpus`` was not specified, Open MPI
defined "processor element" as "core", and therefore the ``--bind-to
core`` caused each process to be bound to both of the cores to which
it was mapped.

In the fourth case, binding is turned off and no bindings are reported.

Open MPI's support for process binding depends on the underlying
operating system.  Therefore, certain process binding options may not
be available on every system.

Process binding can also be set with MCA parameters.  Their usage is
less convenient than that of ``mpirun`` options.  On the other hand,
MCA parameters can be set not only on the mpirun command line, but
alternatively in a system or user ``mca-params.conf`` file or as
environment variables, as described in the :ref:`Setting MCA
Parameters <man1-mpirun-setting-mca-parameters>`.  Some examples
include:

.. list-table::
   :header-rows: 1

   * - Option
     - MCA parameter key
     - Value

   * - ``--map-by core``
     - ``rmaps_base_mapping_policy``
     - ``core``

   * - ``--map-by socket``
     - ``rmaps_base_mapping_policy``
     - ``socket``

   * - ``--rank-by core``
     - ``rmaps_base_ranking_policy``
     - ``core``

   * - ``--bind-to core``
     - ``hwloc_base_binding_policy``
     - ``core``

   * - ``--bind-to socket``
     - ``hwloc_base_binding_policy``
     - ``socket``

   * - ``--bind-to none``
     - ``hwloc_base_binding_policy``
     - ``none``

Rankfiles
^^^^^^^^^

Rankfiles are text files that specify detailed information about how
individual processes should be mapped to nodes, and to which
processor(s) they should be bound.  Each line of a rankfile specifies
the location of one process (for MPI jobs, the process' "rank" refers
to its rank in MPI_COMM_WORLD).  The general form of each line in the
rankfile is:

.. code::

   rank <N>=<hostname> slot=<slot list>

For example:

.. code::

   shell$ cat myrankfile
   rank 0=aa slot=1:0-2
   rank 1=bb slot=0:0,1
   rank 2=cc slot=1-2
   shell$ mpirun -H aa,bb,cc,dd -rf myrankfile ./a.out

Means that:

* Rank 0 runs on node aa, bound to logical socket 1, cores 0-2.
* Rank 1 runs on node bb, bound to logical socket 0, cores 0 and 1.
* Rank 2 runs on node cc, bound to logical cores 1 and 2.

Rankfiles can alternatively be used to specify physical processor
locations. In this case, the syntax is somewhat different. Sockets are
no longer recognized, and the slot number given must be the number of
the physical PU as most OS's do not assign a unique physical
identifier to each core in the node. Thus, a proper physical rankfile
looks something like the following:

.. code::

   shell$ cat myphysicalrankfile
   rank 0=aa slot=1
   rank 1=bb slot=8
   rank 2=cc slot=6

This means that

* Rank 0 will run on node aa, bound to the core that contains physical
  PU 1
* Rank 1 will run on node bb, bound to the core that contains physical
  PU 8
* Rank 2 will run on node cc, bound to the core that contains physical
  PU 6

Rankfiles are treated as logical by default, and the MCA parameter
``rmaps_rank_file_physical`` must be set to 1 to indicate that the
rankfile is to be considered as physical.

The hostnames listed above are "absolute," meaning that actual
resolveable hostnames are specified.  However, hostnames can also be
specified as "relative," meaning that they are specified in relation
to an externally-specified list of hostnames (e.g., by ``mpirun``'s
``--host`` argument, a hostfile, or a job scheduler).

The "relative" specification is of the form ``+n<X>``, where X is an
integer specifying the Xth hostname in the set of all available
hostnames, indexed from 0.  For example:

.. code::

   shell$ cat myrankfile
   rank 0=+n0 slot=1:0-2
   rank 1=+n1 slot=0:0,1
   rank 2=+n2 slot=1-2
   shell$ mpirun -H aa,bb,cc,dd -rf myrankfile ./a.out

All socket/core slot locations are specified as logical indexes.

.. note:: The Open MPI v1.6 series used physical indexes.

You can use tools such as Hwloc's `lstopo(1)` to find the logical
indexes of socket and cores.

Application Context or Executable Program?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To distinguish the two different forms, mpirun looks on the command
line for ``--app`` option.  If it is specified, then the file named on
the command line is assumed to be an application context.  If it is
not specified, then the file is assumed to be an executable program.

Locating Files
^^^^^^^^^^^^^^

If no relative or absolute path is specified for a file, Open MPI will
first look for files by searching the directories specified by the
``--path`` option.  If there is no ``--path`` option set or if the
file is not found at the ``--path`` location, then Open MPI will
search the user's ``PATH`` environment variable as defined on the
source node(s).

If a relative directory is specified, it must be relative to the
initial working directory determined by the specific starter used. For
example when using the ssh starter, the initial directory is ``$HOME``
by default.  Other starters may set the initial directory to the
current working directory from the invocation of ``mpirun``.

.. _man1-mpirun-current-working-directory:

Current Working Directory
^^^^^^^^^^^^^^^^^^^^^^^^^

The ``--wdir`` ``mpirun`` option (and its synonym, ``--wd``) allows
the user to change to an arbitrary directory before the program is
invoked.  It can also be used in application context files to specify
working directories on specific nodes and/or for specific
applications.

If the ``--wdir`` option appears both in a context file and on the
command line, the context file directory will override the command
line value.

If the ``-wdir`` option is specified, Open MPI will attempt to change
to the specified directory on all of the remote nodes. If this fails,
``mpirun`` will abort.

If the ``-wdir`` option is not specified, Open MPI will send the
directory name where ``mpirun`` was invoked to each of the remote
nodes.  The remote nodes will try to change to that directory.  If
they are unable (e.g., if the directory does not exist on that node),
then Open MPI will use the default directory determined by the
starter.

All directory changing occurs before the user's program is invoked; it
does not wait until :ref:`MPI_INIT(3) <mpi_init>` is called.

Standard I/O
^^^^^^^^^^^^

Open MPI directs UNIX standard input to ``/dev/null`` on all processes
except the MPI_COMM_WORLD rank 0 process. The MPI_COMM_WORLD rank 0
process inherits standard input from ``mpirun``.

.. note:: The node that invoked ``mpirun`` need not be the same as the
          node where the MPI_COMM_WORLD rank 0 process resides. Open
          MPI handles the redirection of ``mpirun``'s standard input
          to the rank 0 process.

Open MPI directs UNIX standard output and error from remote nodes to
the node that invoked ``mpirun`` and prints it on the standard
output/error of ``mpirun``.  Local processes inherit the standard
output/error of ``mpirun`` and transfer to it directly.

Thus it is possible to redirect standard I/O for Open MPI applications
by using the typical shell redirection procedure on ``mpirun``.  For
example:

.. code:: sh

   shell$ mpirun -n 2 my_app < my_input > my_output

Note that in this example only the MPI_COMM_WORLD rank 0 process will
receive the stream from ``my_input`` on stdin.  The stdin on all the other
nodes will be tied to ``/dev/null``.  However, the stdout from all nodes
will be collected into the ``my_output`` file.

Signal Propagation
^^^^^^^^^^^^^^^^^^

When ``mpirun`` receives a SIGTERM and SIGINT, it will attempt to kill
the entire job by sending all processes in the job a SIGTERM, waiting
a small number of seconds, then sending all processes in the job a
SIGKILL.

SIGUSR1 and SIGUSR2 signals received by ``mpirun`` are propagated to all
processes in the job.

A SIGTSTOP signal to ``mpirun`` will cause a SIGSTOP signal to be sent
to all of the programs started by ``mpirun`` and likewise a SIGCONT
signal to ``mpirun`` will cause a SIGCONT sent.

Other signals are not currently propagated by ``mpirun``.

Process Termination / Signal Handling
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

During the run of an MPI application, if any process dies abnormally
(either exiting before invoking :ref:`MPI_FINALIZE(3) <mpi_finalize>`,
or dying as the result of a signal), ``mpirun`` will print out an
error message and kill the rest of the MPI application.

User signal handlers should probably avoid trying to cleanup MPI state
(Open MPI is currently not async-signal-safe; see
:ref:`MPI_INIT_THREAD(3) <mpi_init_thread>` for details about
MPI_THREAD_MULTIPLE and thread safety).  For example, if a
segmentation fault occurs in :ref:`MPI_SEND(3) <mpi_send>` (perhaps
because a bad buffer was passed in) and a user signal handler is
invoked, if this user handler attempts to invoke :ref:`MPI_FINALIZE(3)
<mpi_finalize>`, Bad Things could happen since Open MPI was already
"in" MPI when the error occurred.  Since ``mpirun`` will notice that the
process died due to a signal, it is probably not necessary (and
safest) for the user to only clean up non-MPI state.

Process Environment
^^^^^^^^^^^^^^^^^^^

Processes in the MPI application inherit their environment from the
Open RTE daemon upon the node on which they are running.  The
environment is typically inherited from the user's shell.  On remote
nodes, the exact environment is determined by the boot MCA module
used.  The rsh launch module, for example, uses either rsh/ssh to
launch the Open RTE daemon on remote nodes, and typically executes one
or more of the user's shell-setup files before launching the Open RTE
daemon.  When running dynamically linked applications which require
the ``LD_LIBRARY_PATH`` environment variable to be set, care must be
taken to ensure that it is correctly set when booting Open MPI.

See the :ref:`Remote Execution <man1-mpirun-remote-execution>` section
for more details.

.. _man1-mpirun-remote-execution:

Remote Execution
^^^^^^^^^^^^^^^^

Open MPI requires that the ``PATH`` environment variable be set to
find executables on remote nodes (this is typically only necessary in
rsh- or ssh-based environments |mdash| batch/scheduled environments
typically copy the current environment to the execution of remote
jobs, so if the current environment has ``PATH`` and/or
``LD_LIBRARY_PATH`` set properly, the remote nodes will also have it
set properly).  If Open MPI was compiled with shared library support,
it may also be necessary to have the ``LD_LIBRARY_PATH`` environment
variable set on remote nodes as well (especially to find the shared
libraries required to run user MPI applications).

However, it is not always desirable or possible to edit shell startup
files to set ``PATH`` and/or ``LD_LIBRARY_PATH``.  The ``--prefix``
option is provided for some simple configurations where this is not
possible.

The ``--prefix`` option takes a single argument: the base directory on
the remote node where Open MPI is installed.  Open MPI will use this
directory to set the remote ``PATH`` and ``LD_LIBRARY_PATH`` before
executing any Open MPI or user applications.  This allows running Open
MPI jobs without having pre-configured the ``PATH`` and
``LD_LIBRARY_PATH`` on the remote nodes.

Open MPI adds the basename of the current node's ``$bindir`` (the
directory where Open MPI's executables were installed) to the prefix
and uses that to set the ``PATH`` on the remote node.  Similarly, Open
MPI adds the basename of the current node's ``$libdir`` (the directory
where Open MPI's libraries were installed) to the prefix and uses that
to set the ``LD_LIBRARY_PATH`` on the remote node.  For example:

* Local bindir: ``/local/node/directory/bin``
* Local libdir: ``/local/node/directory/lib64``

If the following command line is used:

.. code:: sh

   shell$ mpirun --prefix /remote/node/directory

Open MPI will add ``/remote/node/directory/bin`` to the ``PATH`` and
``/remote/node/directory/lib64`` to the ``LD_LIBRARY_PATH`` on the
remote node before attempting to execute anything.

The ``--prefix`` option is not sufficient if the installation paths on
the remote node are different than the local node (e.g., if ``/lib``
is used on the local node, but ``/lib64`` is used on the remote node),
or if the installation paths are something other than a subdirectory
under a common prefix.

Note that executing ``mpirun`` via an absolute pathname is equivalent
to specifying ``--prefix`` without the last subdirectory in the
absolute pathname to ``mpirun``.  For example:

.. code:: sh

   shell$ /usr/local/bin/mpirun ...

is equivalent to

.. code:: sh

   shell$ mpirun --prefix /usr/local

Exported Environment Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

All environment variables that are named in the form ``OMPI_*`` will
automatically be exported to new processes on the local and remote
nodes.  Environmental parameters can also be set/forwarded to the new
processes using the MCA parameter ``mca_base_env_list``. The ``-x``
option to mpirun has been deprecated, but the syntax of the MCA param
follows that prior example. While the syntax of the ``-x`` option and
MCA param allows the definition of new variables, note that the parser
for these options are currently not very sophisticated |mdash| it does
not even understand quoted values.  Users are advised to set variables
in the environment and use the option to export them; not to define
them.

.. _man1-mpirun-setting-mca-parameters:

Setting MCA Parameters
^^^^^^^^^^^^^^^^^^^^^^

The ``--mca`` switch allows the passing of parameters to various MCA
(Modular Component Architecture) modules.  MCA modules have direct
impact on MPI programs because they allow tunable parameters to be set
at run time (such as which BTL communication device driver to use,
what parameters to pass to that BTL, etc.).

The ``--mca`` switch takes two arguments: ``<key>`` and ``<value>``.
The ``<key>`` argument generally specifies which MCA module will
receive the value.  For example, the ``<key>`` ``btl`` is used to
select which BTL to be used for transporting MPI messages.  The
``<value>`` argument is the value that is passed.  For example:

.. code:: sh

   shell$ mpirun --mca btl tcp,self -n 1 my_mpi_app

This tells Open MPI to use the ``tcp`` and ``self`` BTLs, and to run a
single copy of ``my_mpi_app`` an allocated node.

.. code:: sh

   shell$ mpirun --mca btl self -n 1 my_mpi_app

Tells Open MPI to use the ``self`` BTL, and to run a single copy of
``my_mpi_app`` an allocated node.

The ``--mca`` switch can be used multiple times to specify different
<key> and/or ``<value>`` arguments.  If the same ``<key>`` is
specified more than once, the ``<value>``s are concatenated with a
comma (``,``) separating them.

Note that the ``--mca`` switch is simply a shortcut for setting
environment variables.  The same effect may be accomplished by setting
corresponding environment variables before running ``mpirun``.  The form
of the environment variables that Open MPI sets is:

.. code:: sh

   OMPI_MCA_<key>=<value>

Thus, the ``--mca`` switch overrides any previously set environment
variables.  The ``--mca`` settings similarly override MCA parameters
set in the ``$OPAL_PREFIX/etc/openmpi-mca-params.conf`` or
``$HOME/.openmpi/mca-params.conf`` file.

Unknown ``<key>`` arguments are still set as environment variable --
they are not checked (by mpirun) for correctness.  Illegal or
incorrect ``<value>`` arguments may or may not be reported |mdash| it
depends on the specific MCA module.

To find the available component types under the MCA architecture, or
to find the available parameters for a specific component, use the
ompi_info command.  See the :ref:`ompi_info(1) <man1-ompi_info>` man
page for detailed information on this command.

.. _man1-mpirun-setting-mca-params-from-file:

Setting MCA parameters and environment variables from file
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``--tune`` command line option and its synonym ``--mca``
``mca_base_envar_file_prefix`` allows a user to set MCA parameters and
environment variables with the syntax described below.  This option
requires a single file or list of files separated by "," to follow.

A valid line in the file may contain zero or more ``-x`` or
``--mca``. The following patterns are supported:

* ``--mca var val``
* ``--mca var "val"``
* ``-x var=val``
* ``-x var``

If any argument is duplicated in the file, the last value read will be
used.

MCA parameters and environment specified on the command line
have higher precedence than variables specified in the file.

.. _man1-mpirun-running-as-root:

Running as root
^^^^^^^^^^^^^^^

.. warning:: The Open MPI team **strongly** advises against executing
             ``mpirun`` as the root user.  MPI applications should be
             run as regular (non-root) users.

``mpirun`` will refuse to run as root by default.

To override this default, you can add the ``--allow-run-as-root``
option to the mpirun command line, or you can set the environmental
parameters ``OMPI_ALLOW_RUN_AS_ROOT=1`` and
``OMPI_ALLOW_RUN_AS_ROOT_CONFIRM=1``.  Note that it takes setting two
environment variables to effect the same behavior as
``--allow-run-as-root`` in order to stress the Open MPI team's strong
advice against running as the root user.

After extended discussions with communities who use containers (where
running as the root user is the default), there was a persistent
desire to be able to enable root execution of ``mpirun`` via an
environmental control (vs. the existing ``--allow-run-as-root``
command line parameter).  The compromise of using two environment
variables was reached: it allows root execution via an environmental
control, but it conveys the Open MPI team's strong recommendation
against this behavior.

Exit status
^^^^^^^^^^^

There is no standard definition for what ``mpirun`` should return as
an exit status. After considerable discussion, we settled on the
following method for assigning the ``mpirun`` exit status (note: in
the following description, the "primary" job is the initial
application started by mpirun |mdash| all jobs that are spawned by
that job are designated "secondary" jobs):

* If all processes in the primary job normally terminate with exit
  status 0, ``mpirun`` returns 0.

* If one or more processes in the primary job normally terminate with
  non-zero exit status, ``mpirun`` returns the exit status of the
  process with the lowest MPI_COMM_WORLD rank to have a non-zero
  status.

* If all processes in the primary job normally terminate with exit
  status 0, and one or more processes in a secondary job normally
  terminate with non-zero exit status, ``mpirun``:

  #. Returns the exit status of the process with the lowest
     MPI_COMM_WORLD rank in the lowest jobid to have a non-zero
     status, and
  #. Outputs a message summarizing the exit status of the primary and
     all secondary jobs.

* If the command line option ``--report-child-jobs-separately`` is
  set, we will return *only* the exit status of the primary job. Any
  non-zero exit status in secondary jobs will be reported solely in a
  summary print statement.

By default, the job will abort when any process terminates with
non-zero status. The MCA parameter ``orte_abort_on_non_zero_status``
can be set to "false" (or "0") to cause Open MPI to not abort a job if
one or more processes return a non-zero status. In that situation the
Open MPI records and notes that processes exited with non-zero
termination status to report the appropriate exit status of ``mpirun`` (per
bullet points above).

.. error:: TODO The ``orte_abort...`` name above is definitely wrong for
           Open MPI 5.0.0.

EXAMPLES
--------

Be sure also to see the examples throughout the sections above.

.. code:: sh

   shell$ mpirun -n 4 --mca btl tcp,sm,self prog1

Run 4 copies of ``prog1`` using the ``tcp``, ``sm`` (shared memory),
and ``self`` (process loopback) BTL's for the transport of MPI
messages.


RETURN VALUE
------------

``mpirun`` returns 0 if all processes started by mpirun exit after
calling :ref:`MPI_FINALIZE(3) <mpi_finalize>`.  A non-zero value is
returned if an internal error occurred in mpirun, or one or more
processes exited before calling :ref:`MPI_FINALIZE(3) <mpi_finalize>`.
If an internal error occurred in mpirun, the corresponding error code
is returned.  In the event that one or more processes exit before
calling :ref:`MPI_FINALIZE(3) <mpi_finalize>`, the return value of
the MPI_COMM_WORLD rank of the process that mpirun first notices died
before calling :ref:`MPI_FINALIZE(3) <mpi_finalize>` will be
returned.  Note that, in general, this will be the first process that
died but is not guaranteed to be so.

If the ``--timeout`` command line option is used and the timeout
expires before the job completes (thereby forcing mpirun to kill the
job) mpirun will return an exit status equivalent to the value of
ETIMEDOUT (which is typically 110 on Linux and OS X systems).


.. seealso::
   :ref:`MPI_INIT(3) <mpi_init>`,
   :ref:`MPI_INIT_THREAD(3) <mpi_init_thread>`,
   :ref:`MPI_FINALIZE(3) <mpi_finalize>`,
   :ref:`ompi_info(1) <man1-ompi_info>`
