.. _mpi_init_thread: MPI_Init_thread =============== .. include_body :ref:`MPI_Init_thread` |mdash| Initializes the MPI world model SYNTAX ------ C Syntax ^^^^^^^^ .. code-block:: c #include int MPI_Init_thread(int *argc, char ***argv, int required, int *provided) Fortran Syntax ^^^^^^^^^^^^^^ .. code-block:: fortran USE MPI ! or the older form: INCLUDE 'mpif.h' MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR) INTEGER REQUIRED, PROVIDED, IERROR Fortran 2008 Syntax ^^^^^^^^^^^^^^^^^^^ .. code-block:: fortran USE mpi_f08 MPI_Init_thread(required, provided, ierror) INTEGER, INTENT(IN) :: required INTEGER, INTENT(OUT) :: provided INTEGER, OPTIONAL, INTENT(OUT) :: ierror INPUT PARAMETERS ---------------- * ``argc``: C only: Pointer to the number of arguments. * ``argv``: C only: Argument vector. * ``required``: Desired level of thread support (integer). OUTPUT PARAMETERS ----------------- * ``provided``: Available level of thread support (integer). * ``ierror``: Fortran only: Error status (integer). DESCRIPTION ----------- This routine, or :ref:`MPI_Init`, initializes the MPI world model. Either of these routines must be called before MPI communication routines are called within the MPI world model. The MPI world model can be initialized at most exactly once in the lifetime of an MPI process. This is different than the MPI session model, which can be initialized and finalized multiple times in an MPI process. See :ref:`MPI_Session_init` and :ref:`MPI_Session_finalize`. See `MPI-5.0:11.4.1 `_ for a list of MPI functionality that is available (e.g., even when the MPI world model has not yet initialized or has already been 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`. 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*. Open MPI accepts the C *argc* and *argv* arguments to main, but neither modifies, interprets, nor distributes them: .. code-block:: c #include 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; } :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 any of the values listed below. .. list-table:: :header-rows: 1 * - Value that Open MPI uses - Allowable values (case-insensitive) * - ``MPI_THREAD_SINGLE`` - ``MPI_THREAD_SINGLE``, ``SINGLE``, 0 * - ``MPI_THREAD_FUNNELED`` - ``MPI_THREAD_FUNNELED``, ``FUNNELED``, 1 * - ``MPI_THREAD_SERIALIZED`` - ``MPI_THREAD_SERIALIZED``, ``SERIALIZED``, 2 * - ``MPI_THREAD_MULTIPLE`` - ``MPI_THREAD_MULTIPLE``, ``MULTIPLE``, 3 .. note:: In Open MPI v5.0.8 and prior, only the integer values 0 through 3 were acceptable values for the ``OMPI_MPI_THREAD_LEVEL`` environment variable. Starting with Open MPI v5.0.9, the Open MPI community recomends using one of the string name variants. NOTES ----- The Fortran version does not have provisions for ``argc`` and ``argv`` and 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 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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``MPI_THREAD_MULTIPLE`` support is included if the environment in which Open MPI was built supports threading. You can check the output of :ref:`ompi_info(1) ` 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) shell$ The ``MPI_THREAD_MULTIPLE: yes`` portion of the above output indicates that Open MPI was compiled with ``MPI_THREAD_MULTIPLE`` support. Note that there is a small performance penalty for using ``MPI_THREAD_MULTIPLE`` support; latencies for short messages will be higher as compared to when using ``MPI_THREAD_SINGLE``, for example. ERRORS ------ .. include:: ./ERRORS.rst .. seealso:: * :ref:`MPI_Init` * :ref:`MPI_Initialized` * :ref:`MPI_Finalize` * :ref:`MPI_Finalized` * :ref:`MPI_Session_finalize` * :ref:`MPI_Session_init`