NAME¶
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¶
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)
- 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¶
This routine, or MPI_Init, must be called before any other MPI routine (apart
from MPI_Initialized) is called. 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 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¶
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¶
MPI_THREAD_MULTIPLE support is included if Open MPI was configured with the
--enable-mpi-thread-multiple configure switch. You can check the output of
ompi_info(1) to see if Open MPI has 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).
ERRORS¶
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.
SEE ALSO¶
MPI_Init
MPI_Initialized
MPI_Finalize