SYNTAX

C Syntax

#include <mpi.h>
int MPI_Init_thread(int *argc, char ***argv,
        int required, int *provided)

Fortran Syntax

INCLUDE 'mpif.h'
MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR)
        INTEGER REQUIRED, PROVIDED, IERROR 

C++ Syntax

#include <mpi.h>
int MPI::Init_thread(int& argc, char**& argv, int required)
int MPI::Init_thread(int required)

INPUT PARAMETERS

argc

C/C++ only: Pointer to the number of arguments.

argv

C/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

MPI_Init_thread, as compared to 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 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, except in C++, where it is the return value of the function. 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 MPI_Init_thread with a required value of MPI_THREAD_SINGLE is equivalent to calling MPI_Init.

All MPI programs must contain a call to MPI_Init or MPI_Init_thread. Open MPI accepts the C/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

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 MPI_Init_thread or after an 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

shell$ ompi_info | grep -i thread
          Thread support: posix (mpi: yes, progress: no)
shell$

The "mpi: yes" portion of the above output indicates that Open MPI was compiled with MPI_THREAD_MULTIPLE support.

Note that MPI_THREAD_MULTIPLE support is only lightly tested. It likely does not work for thread-intensive applications. Also note that only the MPI point-to-point communication functions for the BTL's listed below are considered thread safe. Other support functions (e.g., MPI attributes) have not been certified as safe when simultaneously used by multiple threads.

    tcp
    sm
    mx
    elan
    self

Note that Open MPI's thread support is in a fairly early stage; the above devices are likely to work, but the latency is likely to be fairly high. Specifically, efforts so far have concentrated on correctness, not performance (yet).