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MPI_Init_thread - Initializes the MPI execution environment
#include <mpi.h>
int MPI_Init_thread(int *argc, char ***argv,
int required, int *provided)
USE MPI
! or the older form: INCLUDE ’mpif.h’
MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR)
INTEGER REQUIRED, PROVIDED, IERROR
USE mpi_f08
MPI_Init_thread(required, provided, ierror)
INTEGER, INTENT(IN) :: required
INTEGER, INTENT(OUT) :: provided
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
- argc
- C/C++ only: Pointer to the number of arguments.
- argv
- C/C++ only: Argument vector.
- required
- Desired level of thread support (integer).
- provided
- Available level of thread support (integer).
- IERROR
- Fortran only: Error status (integer).
This routine,
or MPI_Init, must be called before most other MPI routines are called. There
are a small number of exceptions, such as MPI_Initialized and MPI_Finalized.
MPI can be initialized at most once; subsequent calls to MPI_Init or MPI_Init_thread
are erroneous.
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 the
C++ binding, 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();
}
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 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
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.
Almost all MPI routines return an error value; C routines
as the value of the function and Fortran routines in the last argument.
C++ functions do not return errors. If the default error handler is set
to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI::Exception object.
Before the error value is
returned, the current MPI error handler is called. By default, this error
handler aborts the MPI job, except for I/O function errors. The error handler
may be changed with MPI_Comm_set_errhandler; the predefined error handler
MPI_ERRORS_RETURN may be used to cause error values to be returned. Note
that MPI does not guarantee that an MPI program can continue past an error.
MPI_Init
MPI_Initialized
MPI_Finalize
MPI_Finalized
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