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MPI_Comm_spawn(3) man page (version 1.5.5)

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MPI_Comm_spawn - Spawns a number of identical binaries.


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’

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

Name of program to be spawned (string, significant only at root).
Arguments to command (array of strings, significant only at root).
Maximum number of processes to start (integer, significant only at root).
A set of key-value pairs telling the runtime system where and how to start the processes (handle, significant only at root).
Rank of process in which previous arguments are examined (integer).
Intracommunicator containing group of spawning processes (handle).

Output Parameter

Intercommunicator between original group and the newly spawned group (handle).
One code per process (array of integers).
Fortran only: Error status (integer).


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
                                          explanation of how this will
be used.
hostfile                        char *    Hostfile containing the hosts
on which
                                          the processes are to be spawned.
                                          the orte_hostfile man page for
                                          explanation of how this will
be used.
add-host                        char *    Add the specified host to the
list of
                                          hosts known to this job and use
                                          for the associated process. This
                                          be used similarly to the -host
add-hostfile                    char *    Hostfile containing hosts to be
                                          to the list of hosts known to
this job and
                                          use it for the associated process.
This will
                                          be used similarly to the -hostfile
wdir                            char *    Directory where the executable
is located. If
                                          files are to be pre-positioned,
then this
                                          location is the desired working
                                          at time of execution - if not
                                          then it will automatically be
set to
ompi_prefix                     char *    Same as the --prefix command line
                                          to mpirun.
ompi_local_slave                bool      If set to true, launch the specified
                                          as a local slave to the calling
                                          The new process will only be
known to the caller,
                                          and will only be able to communicate
with the caller.
ompi_preload_binary             bool      If set to true, pre-position the
                                          executable onto the remote host.
A destination
                                          directory must also be provided.
ompi_preload_files_dest_dir     char *    Target directory where pre-positioned
                                          are to be placed.
ompi_preload_files              char *    A comma-separated list of files
that are to
                                          be pre-positioned in addition
to the executable.
                                          Note that this option does not
depend upon
                                          ompi_preload_binary - files can
be moved
                                          to the target even if an executable
is not moved.
ompi_preload_files_src_dir      char *    Source directory where files
and executables
                                          that are to be pre-positioned
can be found. If
                                          not specified, the current working
                                          will be used.
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
                                          and parent jobs to "hang".
ompi_param                      char *    Pass an OMPI MCA parameter to
the child job.
                                          If that parameter already exists
in the
                                          environment, the value will be
                                          by the provided value.
map_bynode                      bool      If set to true, the processes
are mapped bynode.
                                          If set to false, the processes
are mapped byslot.
                                          By default, mapping is determined
by the default
                                          mapping policy set when the job
was started.

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.


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).


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


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