MPI_Comm_spawn_multiple - Spawn a dynamic MPI process from multiple
MPI_Comm_spawn_multiple(int count, char **commands, char ***argvs,
int *maxprocs, MPI_Info *infos, int root,
MPI_Comm comm, MPI_Comm *intercomm,
count - number of commands (only significant at root)
- commands to be executed (only significant at root)
argvs - arguments for commands (only significant at root)
- max number of processes for each command (only significant at
infos - startup hints for each command
root - rank of process to perform the spawn
comm - parent intracommunicator
- child intercommunicator containing spawned processes
- one code per process
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
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.
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.
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.
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
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.
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.
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.
- No error; MPI routine completed successfully.
- Invalid communicator. A common error is to use a null
communicator in a call (not even allowed in MPI_Comm_rank ).
- Spawn error; one or more of the applications attempting to be
launched failed. Check the returned error code array.
- 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.
- 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.
- Other error; use MPI_Error_string to get more information
about this error code.
- An internal error has been detected. This is fatal. Please
send a bug report to the LAM mailing list (see http://www.lam-
appschema(5), bhost(5), lamboot(1), MPI_Comm_get_parent(3),
MPI_Intercomm_merge(3), MPI_Comm_spawn_multiple(3), MPI_Info_create(3),
MPI_Info_set(3), MPI_Info_delete(3), MPI_Info_free(3), MPI_Init(3),
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).
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.