MPI_Comm_spawn_multiple(3)

NAME
MPI_Comm_spawn_multiple - Spawns multiple binaries, or the same binary with multiple sets of arguments.

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Comm_spawn_multiple(int count, char *array_of_commands[],
	char **array_of_argv[], int array_of_maxprocs[], MPI_Info
	array_of_info[], int root, MPI_Comm comm, MPI_Comm *intercomm,
	int array_of_errcodes[])

Fortran Syntax
INCLUDE 'mpif.h'
MPI_COMM_SPAWN_MULTIPLE(COUNT, ARRAY_OF_COMMANDS, ARRAY_OF_ARGV,
	ARRAY_OF_MAXPROCS, ARRAY_OF_INFO, ROOT, COMM, INTERCOMM,
	ARRAY_OF_ERRCODES, IERROR)
	INTEGER	COUNT, ARRAY_OF_INFO(*), ARRAY_OF_MAXPROCS(*), ROOT,
		COMM, INTERCOMM, ARRAY_OF_ERRCODES(*), IERROR
	CHARACTER*(*) ARRAY_OF_COMMANDS(*), ARRAY_OF_ARGV(COUNT, *)

C++ Syntax
#include <mpi.h>
MPI::Intercomm MPI::Intracomm::Spawn_multiple(int count,
	const char* array_of_commands[], const char** array_of_argv[],
	const int array_of_maxprocs[], const MPI::Info array_of_info[],
	int root, int array_of_errcodes[])
MPI::Intercomm MPI::Intracomm::Spawn_multiple(int count,
	const char* array_of_commands[], const char** array_of_argv[],
	const int array_of_maxprocs[], const MPI::Info array_of_info[],
	int root)

INPUT PARAMETERS

count
    Number of commands (positive integer, significant to MPI only at root -- see NOTES).

array_of_commands
    Programs to be executed (array of strings, significant only at root).

array_of_argv
    Arguments for commands (array of array of strings, significant only at root).

array_of_maxprocs
    Maximum number of processes to start for each command (array of integers, significant only at root).

array_of_info
    Info objects telling the runtime system where and how to start processes (array of handles, significant only at root).

root
    Rank of process in which previous arguments are examined (integer).

comm
    Intracommunicator containing group of spawning processes (handle).

OUTPUT PARAMETERS

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_multiple is identical to MPI_Comm_spawn(3) except that it can specify multiple executables. The first argument, count, indicates the number of executables. The next three arguments are arrays of the corresponding arguments in MPI_Comm_spawn(3). The next argument, array_of_info, is an array of info arguments; however, only the first argument in that array is used. Any subsequent arguments in the array are ignored because an info argument applies to the entire job that is spawned, and cannot be different for each executable in the job. See the INFO ARGUMENTS section for more information.

For the Fortran version of array_of_argv, the element array_of_argv(i,j) is the jth argument to command number i.

In any language, an application may use the constant MPI_ARGVS_NULL (which is likely to be (char ***)0 in C) to specify that no arguments should be passed to any commands. The effect of setting individual elements of array_of_argv to MPI_ARGV_NULL is not defined. To specify arguments for some commands but not others, the commands without arguments should have a corresponding argv whose first element is null ((char *)0 in C and empty string in Fortran).

All of the spawned processes have the same MPI_COMM_WORLD. Their ranks in MPI_COMM_WORLD correspond directly to the order in which the commands are specified in MPI_Comm_spawn_multiple. Assume that m1 processes are generated by the first command, m2 by the second, etc. The processes corresponding to the first command have ranks 0, 1,..., m1-1. The processes in the second command have ranks m1, m1+1, ..., m1+m2-1. The processes in the third have ranks m1+m2, m1+m2+1, ..., m1+m2+m3-1, etc.

The array_of_errcodes argument is 1-dimensional array of size

	 _ count
	\       n ,
	/_ i=1   i

where i is the ith element of array_of_maxprocs. Command number i corresponds to the i contiguous slots in this array from element

                      _              _
	 _ i-1          |   _ i          |
	\       n ,  to |  \      n      | -1
	/_ j=1   i      |  /_ j=1  j     |
                     |_              _|

Error codes are treated as for MPI_Comm_spawn(3).

INFO ARGUMENTS
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.
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.

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.

Note that if any of the info handles have ompi_non_mpi set to true, then all info handles must have it set to true. If some are set to true, but others are set to false (or are unset), MPI_ERR_INFO will be returned.

Note that in "Open MPI", the first array location in array_of_info is applied to all the commands in array_of_commands.

NOTES
The argument count is interpreted by MPI only at the root, as is array_of_argv. Since the leading dimension of array_of_argv is count, a nonpositive value of count at a nonroot node could theoretically cause a runtime bounds check error, even though array_of_argv should be ignored by the subroutine. If this happens, you should explicitly supply a reasonable value of count on the nonroot nodes.

Similar to MPI_Comm_spawn(3), it is the application's responsibility to terminate each individual set of argv in the array_of_argv argument. In C, each argv array is terminated by a NULL pointer. In Fortran, each argv array is terminated by an empty string (note that compilers will not automatically insert this blank string; the application must ensure to have enough space for an empty string entry as the last element of the array).

Other restrictions apply to the array_of_argv parameter; see MPI_Comm_spawn(3)'s description of the argv parameter for more details.

Calling MPI_Comm_spawn(3) many times would create many sets of children with different MPI_COMM_WORLDs, whereas MPI_Comm_spawn_multiple creates children with a single MPI_COMM_WORLD, so the two methods are not completely equivalent. Also if you need to spawn multiple executables, you may get better performance by using MPI_Comm_spawn_multiple instead of calling MPI_Comm_spawn(3) several times.

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(3)
MPI_Comm_get_parent(3)
mpirun(1)