NAME¶
MPI_Comm_spawn - Spawns a number of identical binaries.
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Comm_spawn(char * command, char *argv[], int maxprocs,
MPI_Info info, int root, MPI_Comm comm,
MPI_Comm * intercomm, int array_of_errcodes[])
Fortran Syntax¶
INCLUDE 'mpif.h'
MPI_COMM_SPAWN( COMMAND, ARGV, MAXPROCS, INFO, ROOT, COMM,
INTERCOMM, ARRAY_OF_ERRCODES, IERROR)
CHARACTER*(*) COMMAND, ARGV(*)
INTEGER INFO, MAXPROCS, ROOT, COMM, INTERCOMM,
ARRAY_OF_ERRCODES(*), IERROR
C++ Syntax¶
#include <mpi.h>
MPI::Intercomm MPI::Intracomm::Spawn(const char* command,
const char* argv[], int maxprocs, const MPI::Info& info,
int root, int array_of_errcodes[]) const
MPI::Intercomm MPI::Intracomm::Spawn(const char* command,
const char* argv[], int maxprocs, const MPI::Info& info,
int root) const
- command
- Name of program to be spawned (string, significant only at
root).
- argv
- Arguments to command (array of strings, significant
only at root).
- maxprocs
- Maximum number of processes to start (integer, significant
only at root).
- info
- A set of key-value pairs telling the runtime system where
and how to start the processes (handle, significant only at
root).
- root
- Rank of process in which previous arguments are examined
(integer).
- comm
- Intracommunicator containing group of spawning processes
(handle).
OUTPUT PARAMETER¶
- intercomm
- Intercommunicator between original group and the newly
spawned group (handle).
- array_of_errcodes
- One code per process (array of integers).
- IERROR
- Fortran only: Error status (integer).
DESCRIPTION¶
MPI_Comm_spawn tries to start
maxprocs identical copies of the MPI
program specified by
command, establishing communication with them and
returning an intercommunicator. The spawned processes are referred to as
children. The children have their own MPI_COMM_WORLD, which is separate from
that of the parents. MPI_Comm_spawn is collective over
comm, and also
may not return until MPI_Init has been called in the children. Similarly,
MPI_Init in the children may not return until all parents have called
MPI_Comm_spawn. In this sense, MPI_Comm_spawn in the parents and MPI_Init in
the children form a collective operation over the union of parent and child
processes. The intercommunicator returned by MPI_Comm_spawn contains the
parent processes in the local group and the child processes in the remote
group. The ordering of processes in the local and remote groups is the same as
the as the ordering of the group of the
comm in the parents and of
MPI_COMM_WORLD of the children, respectively. This intercommunicator can be
obtained in the children through the function MPI_Comm_get_parent.
The MPI standard allows an implementation to use the MPI_UNIVERSE_SIZE attribute
of MPI_COMM_WORLD to specify the number of processes that will be active in a
program. Although this implementation of the MPI standard defines
MPI_UNIVERSE_SIZE, it does not allow the user to set its value. If you try to
set the value of MPI_UNIVERSE_SIZE, you will get an error message.
The
command Argument
The
command argument is a string containing the name of a program to be
spawned. The string is null-terminated in C. In Fortran, leading and trailing
spaces are stripped. MPI looks for the file first in the working directory of
the spawning process.
The
argv Argument
argv is an array of strings containing arguments that are passed to the
program. The first element of
argv is the first argument passed to
command, not, as is conventional in some contexts, the command itself.
The argument list is terminated by NULL in C and C++ and an empty string in
Fortran (note that it is the MPI application's responsibility to ensure that
the last entry of the
argv array is an empty string; the compiler will
not automatically insert it). In Fortran, leading and trailing spaces are
always stripped, so that a string consisting of all spaces is considered an
empty string. The constant MPI_ARGV_NULL may be used in C, C++ and Fortran to
indicate an empty argument list. In C and C++, this constant is the same as
NULL.
In C, the MPI_Comm_spawn argument
argv differs from the
argv
argument of
main in two respects. First, it is shifted by one element.
Specifically,
argv[0] of
main contains the name of the program
(given by
command).
argv[1] of
main corresponds to
argv[0] in MPI_Comm_spawn,
argv[2] of
main to
argv[1] of MPI_Comm_spawn, and so on. Second,
argv of
MPI_Comm_spawn must be null-terminated, so that its length can be determined.
Passing an
argv of MPI_ARGV_NULL to MPI_Comm_spawn results in
main receiving
argc of 1 and an
argv whose element 0 is
the name of the program.
The
maxprocs Argument
Open MPI tries to spawn
maxprocs processes. If it is unable to spawn
maxprocs processes, it raises an error of class MPI_ERR_SPAWN. If MPI
is able to spawn the specified number of processes, MPI_Comm_spawn returns
successfully and the number of spawned processes,
m, is given by the
size of the remote group of
intercomm.
A spawn call with the default behavior is called hard. A spawn call for which
fewer than
maxprocs processes may be returned is called soft.
The
info Argument
The
info argument is an opaque handle of type MPI_Info in C, MPI::Info in
C++ and INTEGER in Fortran. It is a container for a number of user-speci ed (
key,value) pairs.
key and
value are strings
(null-terminated char* in C, character*(*) in Fortran). Routines to create and
manipulate the
info argument are described in Section 4.10 of the MPI-2
standard.
For the SPAWN calls,
info provides additional, implementation-dependent
instructions to MPI and the runtime system on how to start processes. An
application may pass MPI_INFO_NULL in C or Fortran. Portable programs not
requiring detailed control over process locations should use MPI_INFO_NULL.
The following keys for
info are recognized in Open MPI. (The reserved
values mentioned in Section 5.3.4 of the MPI-2 standard are not implemented.)
Key Type Description
--- ---- -----------
host char * Host on which the process should be spawned.
See the orte_host man page for an
explanation of how this will be used.
hostfile char * Hostfile containing the hosts on which
the processes are to be spawned. See
the orte_hostfile man page for an
explanation of how this will be used.
wdir char * Directory where the executable is located.
ompi_prefix char * Same as the --prefix command line argument
to mpirun.
ompi_non_mpi bool If set to true, launching a non-MPI
application; the returned communicator
will be MPI_COMM_NULL. Failure to set
this flag when launching a non-MPI
application will cause both the child
and parent jobs to "hang".
bool info keys are actually strings but are evaluated as follows: if the
string value is a number, it is converted to an integer and cast to a boolean
(meaning that zero integers are false and non-zero values are true). If the
string value is (case-insensitive) "yes" or "true", the
boolean is true. If the string value is (case-insensitive) "no" or
"false", the boolean is false. All other string values are
unrecognized, and therefore false.
The
root Argument
All arguments before the
root argument are examined only on the process
whose rank in
comm is equal to
root. The value of these
arguments on other processes is ignored.
The
array_of_errcodes Argument
The
array_of_errcodes is an array of length
maxprocs in which MPI
reports the status of the processes that MPI was requested to start. If all
maxprocs processes were spawned,
array_of_errcodes is filled in
with the value MPI_SUCCESS. If anyof the processes are
not spawned,
array_of_errcodes is filled in with the value MPI_ERR_SPAWN. In C or
Fortran, an application may pass MPI_ERRCODES_IGNORE if it is not interested
in the error codes. In C++ this constant does not exist, and the
array_of_errcodes argument may be omitted from the argument list.
NOTES¶
Completion of MPI_Comm_spawn in the parent does not necessarily mean that
MPI_Init has been called in the children (although the returned
intercommunicator can be used immediately).
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_Comm_spawn_multiple(3)
MPI_Comm_get_parent(3)
mpirun(1)