Ubuntu Manpage: MPI_Comm_spawn - Spawns a number of identical binaries.

Provided by: openmpi-doc_4.1.6-7ubuntu2_all

NAME

       MPI_Comm_spawn - Spawns a number of identical binaries.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Comm_spawn(const char *command, char *argv[], int maxprocs,
            MPI_Info info, int root, MPI_Comm comm,
            MPI_Comm *intercomm, int array_of_errcodes[])

Fortran Syntax

       USE MPI
       ! or the older form: 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

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Comm_spawn(command, argv, maxprocs, info, root, comm, intercomm,
                 array_of_errcodes, ierror)
            CHARACTER(LEN=*), INTENT(IN) :: command, argv(*)
            INTEGER, INTENT(IN) :: maxprocs, root
            TYPE(MPI_Info), INTENT(IN) :: info
            TYPE(MPI_Comm), INTENT(IN) :: comm
            TYPE(MPI_Comm), INTENT(OUT) :: intercomm
            INTEGER :: array_of_errcodes(*)
            INTEGER, OPTIONAL, INTENT(OUT) :: 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 (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-specified (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.
add-host char * Add the specified host to the list of
hosts known to this job and use it for
the associated process. This will be
used similarly to the -host option.
add-hostfile char * Hostfile containing hosts to be added
to the list of hosts known to this job
and use it for the associated
process. This will be used similarly
to the -hostfile option.
wdir char * Directory where the executable is
located. If files are to be
pre-positioned, then this location is
the desired working directory at time
of execution - if not specified, then
it will automatically be set to
ompi_preload_files_dest_dir.
ompi_prefix char * Same as the --prefix command line
argument to mpirun.
ompi_preload_binary bool If set to true, pre-position the
specified executable onto the remote
host. A destination directory must
also be provided.
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_stdin_target char * Comma-delimited list of ranks to
receive stdin when forwarded.
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".
ompi_param char * Pass an OMPI MCA parameter to the
child job. If that parameter already
exists in the environment, the value
will be overwritten by the provided
value.
mapper char * Mapper to be used for this job
map_by char * Mapping directive indicating how
processes are to be mapped (slot,
node, socket, etc.).
rank_by char * Ranking directive indicating how
processes are to be ranked (slot,
node, socket, etc.).
bind_to char * Binding directive indicating how
processes are to be bound (core, slot,
node, socket, etc.).
path char * List of directories to search for
the executable
npernode char * Number of processes to spawn on
each node of the allocation
pernode bool Equivalent to npernode of 1
ppr char * Spawn specified number of processes
on each of the identified object type
env char * Newline-delimited list of envars to
be passed to the spawned procs

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.