docs: numerous updates to MPI_Init[_thread] man pages

jsquyres · jsquyres · commit 027fda50f2c5 · 2025-06-21T15:48:16.000-04:00
Including, but not limited to:

* Add references to the MPI world model (since the sessions model now
  exists), and update outdated text that was no longer entirely
  correct now that the sessions model exists
* Numerous formatting updates
* Slightly improve the C code examples
* Describe the mathematical relationship between the various
  MPI_THREAD_* constants
  * Note that the mathematical relationships render nicely in HTML,
    but don't render entirely properly in nroff.  This commit author
    is of the opinion that the nroff rendering is currently "good
    enough", and some Sphinx maintainer will fix it someday.
* Add descriptions about the $OMPI_MPI_THREAD_LEVEL env variable and
  how it is used
* Added more seealso links

Signed-off-by: Jeff Squyres &lt;jeff@squyres.com&gt;
diff --git a/docs/man-openmpi/man3/MPI_Init.3.rst b/docs/man-openmpi/man3/MPI_Init.3.rst
@@ -6,7 +6,7 @@ MPI_Init
 
 .. include_body
 
-:ref:`MPI_Init` |mdash| Initializes the MPI execution environment
+:ref:`MPI_Init` |mdash| Initializes the MPI world model
 
 .. The following file was automatically generated
 .. include:: ./bindings/mpi_init.rst
@@ -23,35 +23,52 @@ OUTPUT PARAMETER
 DESCRIPTION
 -----------
 
-This routine, or :ref:`MPI_Init_thread`, must be called before most other MPI
-routines are called. There are a small number of errors, such as
-:ref:`MPI_Initialized` and :ref:`MPI_Finalized`. MPI can be initialized at most once;
-subsequent calls to :ref:`MPI_Init` or :ref:`MPI_Init_thread` are erroneous.
+When using the MPI world model, this routine, or
+:ref:`MPI_Init_thread`, must be called before most other MPI routines
+are called. There are a small number of routines that can be invoked
+before :ref:`MPI_Init` or :ref:`MPI_Init_thread`, such as
+:ref:`MPI_Initialized` and :ref:`MPI_Finalized`.
+
+The MPI world model can be initialized at most once; subsequent calls
+to :ref:`MPI_Init` or :ref:`MPI_Init_thread` are erroneous.
+
+Alternatively, instead of the MPI world model, MPI applications can
+use the sessions model; see :ref:`MPI_Session_init`.
 
-All MPI programs must contain a call to :ref:`MPI_Init` or :ref:`MPI_Init_thread`.
 Open MPI accepts the C *argc* and *argv* arguments to main, but neither
 modifies, interprets, nor distributes them:
 
 .. code-block:: c
 
-   /* declare variables */
-   MPI_Init(&argc, &argv);
-   /* parse arguments */
-   /* main program */
-   MPI_Finalize();
+   #include <mpi.h>
+
+   int main(int argv, char *argv[]) {
+       MPI_Init(&argc, &argv);
+       /* ...body of main MPI pogram... */
+       MPI_Finalize();
+       return 0;
+   }
 
+By default, :ref:`MPI_Init` is effectively equivalent to invoking
+:ref:`MPI_Init_thread` with a *required* value of
+``MPI_THREAD_SINGLE``.  However, if the ``OMPI_MPI_THREAD_LEVEL``
+environment variable is set to a valid value when :ref:`MPI_Init` is
+invoked, it is equivalent to invoking :ref:`MPI_Init_thread` with
+*required* set to the integer value of the ``OMPI_MPI_THREAD_LEVEL``
+environment variable.  See :ref:`MPI_Init_thread` for more details.
 
 NOTES
 -----
 
 The Fortran version does not have provisions for *argc* and *argv* and
 takes only IERROR.
 
-The MPI Standard does not say what a program can do before an :ref:`MPI_Init`
-or after an :ref:`MPI_Finalize`. In the Open MPI implementation, it should do
-as little as possible. In particular, avoid anything that changes the
-external state of the program, such as opening files, reading standard
-input, or writing to standard output.
+The MPI Standard does not specify what a program using the MPI world
+model can do before invoking :ref:`MPI_Init` or :ref:`MPI_Init_thread`
+or after invoking :ref:`MPI_Finalize`. In the Open MPI implementation,
+it should do as little as possible. In particular, avoid anything that
+changes the external state of the program, such as opening files,
+reading standard input, or writing to standard output.
 
 
 ERRORS
@@ -64,3 +81,5 @@ ERRORS
    * :ref:`MPI_Initialized`
    * :ref:`MPI_Finalize`
    * :ref:`MPI_Finalized`
+   * :ref:`MPI_Session_finalize`
+   * :ref:`MPI_Session_init`
diff --git a/docs/man-openmpi/man3/MPI_Init_thread.3.rst b/docs/man-openmpi/man3/MPI_Init_thread.3.rst
@@ -6,7 +6,7 @@ MPI_Init_thread
 
 .. include_body
 
-:ref:`MPI_Init_thread` |mdash| Initializes the MPI execution environment
+:ref:`MPI_Init_thread` |mdash| Initializes the MPI world model
 
 .. The following file was automatically generated
 .. include:: ./bindings/mpi_init_thread.rst
@@ -25,64 +25,137 @@ OUTPUT PARAMETERS
 DESCRIPTION
 -----------
 
-This routine, or :ref:`MPI_Init`, must be called before most other MPI routines
-are called. There are a small number of exceptions, such as
-:ref:`MPI_Initialized` and :ref:`MPI_Finalized`. MPI can be initialized at most once;
-subsequent calls to :ref:`MPI_Init` or :ref:`MPI_Init_thread` are erroneous.
+When using the MPI world model, this routine, or
+:ref:`MPI_Init_thread`, must be called before most other MPI routines
+are called. There are a small number of routines that can be invoked
+before :ref:`MPI_Init` or :ref:`MPI_Init_thread`, such as
+:ref:`MPI_Initialized` and :ref:`MPI_Finalized`.
 
-:ref:`MPI_Init_thread`, as compared to :ref:`MPI_Init`, has a provision to request a
-certain level of thread support in *required*:
+The MPI world model can be initialized at most once; subsequent calls
+to :ref:`MPI_Init` or :ref:`MPI_Init_thread` are erroneous.
 
-MPI_THREAD_SINGLE
-   Only one thread will execute.
+Alternatively, instead of the MPI world model, MPI applications can
+use the sessions model; see :ref:`MPI_Session_init`.
 
-MPI_THREAD_FUNNELED
-   If the process is multithreaded, only the thread that called
-   :ref:`MPI_Init_thread` will make MPI calls.
+Upon return, the level of thread support available to the program is
+set in *provided*. In Open MPI, the value is dependent on how the
+library was configured and built. Note that there is no guarantee that
+*provided* will be greater than or equal to *required*.
 
-MPI_THREAD_SERIALIZED
-   If the process is multithreaded, only one thread will make MPI
-   library calls at one time.
+Open MPI accepts the C *argc* and *argv* arguments to main, but
+neither modifies, interprets, nor distributes them:
 
-MPI_THREAD_MULTIPLE
-   If the process is multithreaded, multiple threads may call MPI at
-   once with no restrictions.
+.. code-block:: c
 
-The level of thread support available to the program is set in
-*provided*. In Open MPI, the value is dependent on how the library was
-configured and built. Note that there is no guarantee that *provided*
-will be greater than or equal to *required*.
+   #include <mpi.h>
 
-Also note that calling :ref:`MPI_Init_thread` with a *required* value of
-MPI_THREAD_SINGLE is equivalent to calling :ref:`MPI_Init`.
+   int main(int argv, char *argv[]) {
+       int provided;
+       MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided);
+       /* ...body of main MPI pogram... */
+       MPI_Finalize();
+       return 0;
+   }
 
-All MPI programs must contain a call to :ref:`MPI_Init` or :ref:`MPI_Init_thread`.
-Open MPI accepts the C *argc* and *argv* arguments to main, but neither
-modifies, interprets, nor distributes them:
 
-.. code-block:: c
+:ref:`MPI_Init_thread` has both a direct and an indirect mechanism to
+request a specific level of thread support.  :ref:`MPI_Init` only has
+an indirect mechanism to request a specific level of thread support.
+
+Direct request of thread level
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:ref:`MPI_Init_thread` has the *required* parameter, which can be set
+to any one of the following constants (from ``mpi.h``):
+
+* ``MPI_THREAD_SINGLE``: Indicating that only one thread will execute.
+
+* ``MPI_THREAD_FUNNELED``: Indicating that if the process is
+  multithreaded, only the thread that called :ref:`MPI_Init_thread`
+  will make MPI calls.
+
+* ``MPI_THREAD_SERIALIZED``: Indicating that if the process is
+  multithreaded, only one thread will make MPI library calls at one
+  time.
+
+* ``MPI_THREAD_MULTIPLE``: Indicating that if the process is
+  multithreaded, multiple threads may call MPI at once with no
+  restrictions.
+
+The values of these constants adhere to the following relationships:
+
+.. math::
+   :nowrap:
+
+   \begin{eqnarray}
+       MPI\_THREAD\_SINGLE     & < & MPI\_THREAD\_FUNNELED \\
+       MPI\_THREAD\_FUNNELED   & < & MPI\_THREAD\_SERIALIZED \\
+       MPI\_THREAD\_SERIALIZED & < & MPI\_THREAD\_MULTIPLE \\
+   \end{eqnarray}
+
+Indirect request of thread level
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Both :ref:`MPI_Init_thread` and :ref:`MPI_Init` support an indirect
+method of indicating the required thread level: setting the
+``OMPI_MPI_THREAD_LEVEL`` environment variable:
+
+* If the ``OMPI_MPI_THREAD_LEVEL`` environment variable is set at the
+  time :ref:`MPI_Init` is invoked, it behaves as if
+  :ref:`MPI_Init_thread` was invoked with the corresponding
+  ``MPI_THREAD_*`` constant value passed via the *required* parameter.
+
+* If the ``OMPI_MPI_THREAD_LEVEL`` environment variable is set at the
+  time :ref:`MPI_Init_thread` is invoked, the ``MPI_THREAD_*``
+  constant value corresponding to the environment variable value
+  overrides the value passed via the *required* parameter.
+
+The ``OMPI_MPI_THREAD_LEVEL`` environment variable can be set to one
+of several different values:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Value that Open MPI uses
+     - Allowable values (case-insensitive)
+
+   * - ``MPI_THREAD_SINGLE``
+     - ``MPI_THREAD_SINGLE``, ``THREAD_SINGLE``, ``SINGLE``, 0
+
+   * - ``MPI_THREAD_FUNNELED``
+     - ``MPI_THREAD_FUNNELED``, ``THREAD_FUNNELED``, ``FUNNELED``, 1
+
+   * - ``MPI_THREAD_SERIALIZED``
+     - ``MPI_THREAD_SERIALIZED``, ``THREAD_SERIALIZED``,
+       ``SERIALIZED``, 2
+
+   * - ``MPI_THREAD_MULTIPLE``
+     - ``MPI_THREAD_MULTIPLE``, ``THREAD_MULTIPLE``, ``MULTIPLE``, 3
 
-   /* declare variables */
-   MPI_Init_thread(&argc, &argv, req, &prov);
-   /* parse arguments */
-   /* main program */
-   MPI_Finalize();
+.. note:: Prior to Open MPI v6.0.0, only the integer values 0 through
+          3 were acceptable values for the ``OMPI_MPI_THREAD_LEVEL``
+          environment variable.
 
+          Starting with Open MPI v6.0.0, the Open MPI community
+          recomends using one of the string name variants so that it
+          can be correctly mapped to the corresponding Open MPI ABI
+          value or the MPI Standard ABI value, as relevant.
 
 NOTES
 -----
 
 The Fortran version does not have provisions for ``argc`` and ``argv`` and
-takes only ``IERROR``.
+takes only ``REQUIRED``, ``PROVIDED``, and ``IERROR``.
 
 It is the caller's responsibility to check the value of ``provided``, as
 it may be less than what was requested in ``required``.
 
-The MPI Standard does not say what a program can do before an
-:ref:`MPI_Init_thread` or after an :ref:`MPI_Finalize`. In the Open MPI
-implementation, it should do as little as possible. In particular, avoid
-anything that changes the external state of the program, such as opening
-files, reading standard input, or writing to standard output.
+The MPI Standard does not specify what a program using the MPI world
+model can do before invoking :ref:`MPI_Init` or :ref:`MPI_Init_thread`
+or after invoking :ref:`MPI_Finalize`. In the Open MPI implementation,
+it should do as little as possible. In particular, avoid anything that
+changes the external state of the program, such as opening files,
+reading standard input, or writing to standard output.
 
 
 MPI_THREAD_MULTIPLE Support
@@ -93,7 +166,7 @@ Open MPI was built supports threading. You can check the output of
 :ref:`ompi_info(1) <man1-ompi_info>` to see if Open MPI has
 ``MPI_THREAD_MULTIPLE`` support:
 
-::
+.. code-block:: bash
 
    shell$ ompi_info | grep "Thread support"
              Thread support: posix (MPI_THREAD_MULTIPLE: yes, OPAL support: yes, OMPI progress: no, Event lib: yes)
@@ -117,3 +190,5 @@ ERRORS
    * :ref:`MPI_Initialized`
    * :ref:`MPI_Finalize`
    * :ref:`MPI_Finalized`
+   * :ref:`MPI_Session_finalize`
+   * :ref:`MPI_Session_init`
diff --git a/docs/man-openmpi/man3/MPI_Session_finalize.3.rst b/docs/man-openmpi/man3/MPI_Session_finalize.3.rst
@@ -25,7 +25,7 @@ DESCRIPTION
 
 :ref:`MPI_Session_finalize` releases all MPI state associated with the supplied
 session. Every instantiated session must be finalized using
-:ref:`MPI_Session_finalize`. The handle session is set to MPI_SESSION_NULL by
+:ref:`MPI_Session_finalize`. The handle session is set to ``MPI_SESSION_NULL`` by
 the call.
 
 NOTES
@@ -52,4 +52,5 @@ ERRORS
 
 .. include:: ./ERRORS.rst
 
-.. seealso:: :ref:`MPI_Session_init`
+.. seealso::
+   * :ref:`MPI_Session_init`
diff --git a/docs/man-openmpi/man3/MPI_Session_init.3.rst b/docs/man-openmpi/man3/MPI_Session_init.3.rst
@@ -46,4 +46,19 @@ ERRORS
 
 .. include:: ./ERRORS.rst
 
-.. seealso:: :ref:`MPI_Session_get_num_psets` MPI_Session_group_from_pset
+.. seealso::
+   * :ref:`MPI_Group_from_session_pset`
+   * :ref:`MPI_Session_c2f`
+   * :ref:`MPI_Session_call_errhandler`
+   * :ref:`MPI_Session_create_errhandler`
+   * :ref:`MPI_Session_f2c`
+   * :ref:`MPI_Session_finalize`
+   * :ref:`MPI_Session_get_errhandler`
+   * :ref:`MPI_Session_get_info`
+   * :ref:`MPI_Session_get_nth_pset`
+   * :ref:`MPI_Session_get_num_psets`
+   * :ref:`MPI_Session_get_pset_info`
+   * :ref:`MPI_Session_init`
+   * :ref:`MPI_Session_set_errhandler`
+   * :ref:`MPI_T_pvar_session_create`
+   * :ref:`MPI_T_pvar_session_free`
