MPI_Comm_spawn_multiple

MPI_Comm_spawn_multiple - 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

OUTPUT PARAMETERS

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.

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. Both groups can decide to merge the intercommunicator into an intracommunicator (with the MPI_Comm_merge () function).

Info keys

Key:	Description:
file	The spawning can be based on the configuration of a process group .pg file. Since a .pg file fully specifies how many, which, and where processes should be spawned no other info key is read if the file key is present. The .pg file needs only to be present at the root of the spawn.
host	The host name (Windows Computer Name) as specified in the cluster configuration file of the machine to host the processes to be spawned. You are required specify the host key in every info argument if you specify it in one.
path	Value is the directory where csWMPI should look for the executable(s). You can specify multiple paths separated by semicolon in each info object.

In case the file key is used in any info object the corresponding command, argv, and maxproc are all ignored. Note that even though maxprocs is ignored, errcodes must still be an array long enough to hold an integer error code for every process that tried to launch, or be the MPI constant MPI_ERRCODES_IGNORE.

Also note that this "file" key is not portable to other MPI implementations.

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.

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.

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.

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.