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

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

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-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.
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_local_slave bool If set to true, launch the specified process
as a local slave to the calling process.
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 specified
executable onto the remote host. A destination
directory must also be provided.
ompi_preload_files_dest_dir char * Target directory where pre-positioned files
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 directory
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 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.
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.

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.