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MPI_Init_thread

:ref:`MPI_Init_thread` - Initializes the MPI execution environment

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Init_thread(int *argc, char ***argv,
     int required, int *provided)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR)
     INTEGER REQUIRED, PROVIDED, IERROR

Fortran 2008 Syntax

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`, 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.

:ref:`MPI_Init_thread`, as compared to :ref:`MPI_Init`, has a provision to request a certain level of thread support in required:

MPI_THREAD_SINGLE
Only one thread will execute.
MPI_THREAD_FUNNELED
If the process is multithreaded, only the thread that called :ref:`MPI_Init_thread` will make MPI calls.
MPI_THREAD_SERIALIZED
If the process is multithreaded, only one thread will make MPI library calls at one time.
MPI_THREAD_MULTIPLE
If the process is multithreaded, multiple threads may call MPI at once with no restrictions.

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.

Also note that calling :ref:`MPI_Init_thread` with a required value of MPI_THREAD_SINGLE is equivalent to calling :ref:`MPI_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:

/* declare variables */
MPI_Init_thread(&argc, &argv, req, &prov);
/* parse arguments */
/* main program */
MPI_Finalize();

NOTES

The Fortran version does not have provisions for argc and argv and takes only 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.

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) <man1-ompi_info>` to see if Open MPI has MPI_THREAD_MULTIPLE support:

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

.. seealso::
   * :ref:`MPI_Init`
   * :ref:`MPI_Initialized`
   * :ref:`MPI_Finalize`
   * :ref:`MPI_Finalized`