MPI_Comm_spawn(3)

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_ERRORCODES(*), 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

INPUT PARAMETERS

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. 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)