man MPI_Comm_spawn_multiple (3): Spawn a dynamic MPI process from multiple executables

SYNOPSIS

#include <mpi.h>
int
MPI_Comm_spawn_multiple(int count, char **commands, char ***argvs,
int *maxprocs, MPI_Info *infos, int root,
MPI_Comm comm, MPI_Comm *intercomm,
int *errcodes)

INPUT PARAMETERS

count: - number of commands (only significant at root)
commands: - commands to be executed (only significant at root)
argvs: - arguments for commands (only significant at root)
maxprocs: - max number of processes for each command (only significant at root)
infos: - startup hints for each command
root: - rank of process to perform the spawn
comm: - parent intracommunicator

OUTPUT PARAMETERS

intercomm: - child intercommunicator containing spawned processes
errcodes: - one code per process

DESCRIPTION

A group of processes can create another group of processes with MPI_Comm_spawn_multiple . This function is a collective operation over the parent communicator. The child group starts up like any MPI application. The processes must begin by calling MPI_Init , after which the pre-defined communicator, MPI_COMM_WORLD , may be used. This world communicator contains only the child processes. It is distinct from the MPI_COMM_WORLD of the parent processes.

MPI_Comm_spawn_multiple is used to manually specify a group of different executables and arguments to spawn. MPI_Comm_spawn is used to specify one executable and set of arguments (although a LAM/MPI appschema(5) can be provided to MPI_Comm_spawn via the "file" info key).

Communication With Spawned Processes

The natural communication mechanism between two groups is the intercommunicator. The second communicator argument to MPI_Comm_spawn_multiple returns an intercommunicator whose local group contains the parent processes (same as the first communicator argument) and whose remote group contains child processes. The child processes can access the same intercommunicator by using the MPI_Comm_get_parent call. The remote group size of the parent communicator is zero if the process was created by mpirun (1) instead of one of the spawn functions. Both groups can decide to merge the intercommunicator into an intracommunicator (with the MPI_Intercomm_merge () function) and take advantage of other MPI collective operations. They can then use the merged intracommunicator to create new communicators and reach other processes in the MPI application.

Resource Allocation

Note that no MPI_Info keys are recognized by this implementation of MPI_Comm_spawn_multiple . To use the "file" info key to specify an appschema(5), use LAM's MPI_Comm_spawn . This may be preferable to MPI_Comm_spawn_multiple because it allows the arbitrary specification of what nodes and/or CPUs should be used to launch jobs (either SPMD or MPMD). See MPI_Comm_spawn(3) for more details.

The value of MPI_INFO_NULL should be given for each value in infos (the infos array is not currently examined by LAM/MPI, so specifying non-NULL values for the array values is not harmful). LAM schedules the given number of processes onto LAM nodes by starting with CPU 0 (or the lowest numbered CPU), and continuing through higher CPU numbers, placing one process on each CPU. If the process count is greater than the CPU count, the procedure repeats.

Process Terminiation

Note that the process[es] spawned by MPI_COMM_SPAWN (and MPI_COMM_SPAWN_MULTIPLE ) effectively become orphans. That is, the spawnning MPI application does not wait for the spawned application to finish. Hence, there is no guarantee the spawned application has finished when the spawning completes. Similarly, killing the spawning application will also have no effect on the spawned application.

User applications can effect this kind of behavior with MPI_BARRIER between the spawning and spawned processed before MPI_FINALIZE .

Note that lamclean will kill *all* MPI processes.

Process Count

The maxprocs array parameter to MPI_Comm_spawn_multiple specifies the exact number of processes to be started. If it is not possible to start the desired number of processes, MPI_Comm_spawn_multiple will return an error code. Note that even though maxprocs is only relevant on the root, all ranks must have an errcodes array long enough to handle an integer error code for every process that tries to launch, or give MPI constant MPI_ERRCODES_IGNORE for the errcodes argument. While this appears to be a contradiction, it is per the MPI-2 standard. :-\

Frequently, an application wishes to chooses a process count so as to fill all processors available to a job. MPI indicates the maximum number of processes recommended for a job in the pre-defined attribute, MPI_UNIVERSE_SIZE , which is cached on MPI_COMM_WORLD .

The typical usage is to subtract the value of MPI_UNIVERSE_SIZE from the number of processes currently in the job and spawn the difference. LAM sets MPI_UNIVERSE_SIZE to the number of CPUs in the user's LAM session (as defined in the boot schema [bhost(5)] via lamboot (1)).

See MPI_Init(3) for other pre-defined attributes that are helpful when spawning.

Locating an Executable Program

The executable program file must be located on the node(s) where the process(es) will run. On any node, the directories specified by the user's PATH environment variable are searched to find the program.

All MPI runtime options selected by mpirun (1) in the initial application launch remain in effect for all child processes created by the spawn functions.

Command-line Arguments

The argvs array parameter to MPI_Comm_spawn_multiple should not contain the program name since it is given in the first parameter. The command line that is passed to the newly launched program will be the program name followed by the strings in corresponding entry in the argvs array.

USAGE WITH IMPI EXTENSIONS

The IMPI standard only supports MPI-1 functions. Hence, this function is currently not designed to operate within an IMPI job.

ERRORS

If an error occurs in an MPI function, the current MPI error handler is called to handle it. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Errhandler_set ; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned (in C and Fortran; this error handler is less useful in with the C++ MPI bindings. The predefined error handler MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value needs to be recovered). Note that MPI does not guarantee that an MPI program can continue past an error.

All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error value; C routines as the value of the function and Fortran routines in the last argument. The C++ bindings for MPI do not return error values; instead, error values are communicated by throwing exceptions of type MPI::Exception (but not by default). Exceptions are only thrown if the error value is not MPI::SUCCESS .

Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error, there will be no way to recover what the actual error value was.

MPI_SUCCESS: - No error; MPI routine completed successfully.
MPI_ERR_COMM: - Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank ).
MPI_ERR_SPAWN: - Spawn error; one or more of the applications attempting to be launched failed. Check the returned error code array.
MPI_ERR_ARG: - Invalid argument. Some argument is invalid and is not identified by a specific error class. This is typically a NULL pointer or other such error.
MPI_ERR_ROOT: - Invalid root. The root must be specified as a rank in the communicator. Ranks must be between zero and the size of the communicator minus one.
MPI_ERR_OTHER: - Other error; use MPI_Error_string to get more information about this error code.
MPI_ERR_INTERN: - An internal error has been detected. This is fatal. Please send a bug report to the LAM mailing list (see http://www.lam-mpi.org/contact.php ).

MORE INFORMATION

For more information, please see the official MPI Forum web site, which contains the text of both the MPI-1 and MPI-2 standards. These documents contain detailed information about each MPI function (most of which is not duplicated in these man pages).

http://www.mpi-forum.org/

ACKNOWLEDGEMENTS

The LAM Team would like the thank the MPICH Team for the handy program to generate man pages ("doctext" from ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial formatting, and some initial text for most of the MPI-1 man pages.

LOCATION

spawnmult.c