17.2.232. MPI_Init_thread

MPI_Init_thread — Initializes the MPI world model

17.2.232.1. SYNTAX

17.2.232.1.1. C Syntax

#include <mpi.h>

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

17.2.232.1.2. Fortran Syntax

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

17.2.232.1.3. 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

17.2.232.2. INPUT PARAMETERS

argc: C only: Pointer to the number of arguments.
argv: C only: Argument vector.
required: Desired level of thread support (integer).

17.2.232.3. OUTPUT PARAMETERS

provided: Available level of thread support (integer).
ierror: Fortran only: Error status (integer).

17.2.232.4. DESCRIPTION

This routine, or 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 MPI_Session_init and 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 MPI_Init or MPI_Init_thread are erroneous.

Alternatively, instead of the MPI world model, MPI applications can use the sessions model; see 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:

#include <mpi.h>

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;
}

MPI_Init_thread has both a direct and an indirect mechanism to request a specific level of thread support. MPI_Init only has an indirect mechanism to request a specific level of thread support.

17.2.232.4.1. Direct request of thread level

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 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:

\begin{eqnarray} MPI\_THREAD\_SINGLE & < & MPI\_THREAD\_FUNNELED \\ MPI\_THREAD\_FUNNELED & < & MPI\_THREAD\_SERIALIZED \\ MPI\_THREAD\_SERIALIZED & < & MPI\_THREAD\_MULTIPLE \\ \end{eqnarray}

17.2.232.4.2. Indirect request of thread level

Both MPI_Init_thread and 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 MPI_Init is invoked, it behaves as if 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 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.

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.

17.2.232.5. 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 MPI_Init or MPI_Init_thread or after invoking 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.

17.2.232.5.1. 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 ompi_info(1) 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.

17.2.232.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.
MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.
MPI_ERRORS_RETURN Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.