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RedHat 9 (Linux i386) - man page for mpi_comm_spawn (redhat section 3)

MPI_Comm_spawn(3)			     LAM/MPI				MPI_Comm_spawn(3)

       MPI_Comm_spawn -  Spawn a dynamic MPI process

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

	      - Name of program to spawn (only significant at root)
       argv   - arguments to command (only significant at root)
	      - max number of processes to start (only significant at root)
       info   - startup hints
       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
	.  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

       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 sec-
       ond communicator argument to MPI_Comm_spawn 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_Comm_merge  function) and take advantage of other MPI collective operations.  They can
       then use the merged intracommunicator to create new communicators  and  reach  other  pro-
       cesses in the MPI application.

       Resource Allocation

       As  requested  by  users, LAM/MPI offers one MPI_Info key - "file".  The value of this key
       can be the filename of an appschema(1).	This allows the programmer to  specify	an  arbi-
       trary  set  of  LAM  CPUs or nodes to spawn MPI processes on.  In this case, only the app-
       schema is used to spawn the application; command , argv , and  maxprocs	are  all  ignored
       (even  at the root).  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 MPI_Comm_spawn_multiple does not accept the "file" info key.  As such,
       the "file" info key to MPI_Comm_spawn is mainly intended to spawn MPMD applications and/or
       specify an arbitrary number of nodes to run on.

       Also  note  that  this  "file"  key  is not portable to other MPI implementations; it is a
       LAM/MPI-specific info key.  If specifying exact LAM nodes or CPUs is not necessary,  users
       should probably use MPI_Comm_spawn_multiple to make their program more portable.

       If  the	"file"	info key is not used, the value of MPI_INFO_NULL should be given for info
       (all other keys are ignored, anyway - there is no harm in providing other keys).  In  this
       case,  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

       Process Count

       The maxprocs parameter to MPI_Comm_spawn specifies the exact number  of	processes  to  be
       started.   If  it is not possible to start the desired number of processes, MPI_Comm_spawn
       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 stan-
       dard.  :-\

       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)).

       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 argv parameter to MPI_Comm_spawn 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 argv

       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 excep-
       tions 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-mpi.org/contact.php ).

       appschema(5),	bhost(5),    lamboot(1),    MPI_Comm_get_parent(3),    MPI_Comm_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), mpirun(1)

       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  for-
       matting, and some initial text for most of the MPI-1 man pages.


LAM/MPI 6.5.8				    11/10/2002				MPI_Comm_spawn(3)

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