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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)
INCLUDE ’mpif.h’
MPI_INIT_THREAD(REQUIRED, PROVIDED, IERROR)
INTEGER REQUIRED, PROVIDED, IERROR
#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).
- provided
- Available level of thread support (integer).
- IERROR
- Fortran only: Error status (integer).
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();
}
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 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).
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
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