MPI_Comm_spawn - Spawn a dynamic MPI process
#include <mpi.h> int MPI_Comm_spawn(char* command, char** argv, int maxprocs, MPI_Info info,
int root, MPI_Comm comm, MPI_Comm *intercomm,
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 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 "lam_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 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.
LAM/MPI offers some MPI_Info keys for the placement of spawned applications. Keys are looked for in the order listed below. The first key that is found is used; any remaining keys are ignored.
The value of this key can be the filename of an appschema(1). This allows the programmer to specify an arbitrary set of LAM CPUs or nodes to spawn MPI processes on. In this case, only the appschema 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 "lam_spawn_file" info key. As such, the "lam_spawn_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 "lam_spawn_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.
This key is a synonym for "lam_spawn_file". Since "file" is not a LAM-specific name, yet this key carries a LAM-specific meaning, its use is deprecated in favor of "lam_spawn_file".
The value of this key is a string representing a LAM CPU or node (using standard LAM nomenclature -- see mpirun(1)) to begin spawning on. The use of this key allows the programmer to indicate which node/CPU for LAM to start spawning on without having to write out a temporary app schema file.
The CPU number is relative to the boot schema given to lamboot(1). Only a single LAM node/CPU may be specified, such as "n3" or "c1". If a node is specified, LAM will spawn one MPI process per node. If a CPU is specified, LAM will scedule one MPI process per CPU. An error is returned if "N" or "C" is used.
Note that LAM is not involved with run-time scheduling of the MPI process -- LAM only spawns processes on indicated nodes. The operating system schedules these processes for executation just like any other process. No attempt is made by LAM to bind processes to CPUs. Hence, the "cX" nomenclature is just a convenicence mechanism to inidicate how many MPI processes should be spawned on a given node; it is not indicative of operating system scheduling.
For "nX" values, the first MPI process will be spawned on the indicated node. The remaining (maxprocs - 1) MPI processes will be spawned on successive nodes. Specifically, if X is the starting node number, process i will be launched on "nK", where K = ((X + i) % total_nodes). LAM will modulus the node number with the total number of nodes in the current LAM universe to prevent errors, thereby creating a "wraparound" effect. Hence, this mechanism can be used for round-robin scheduling, regardless of how many nodes are in the LAM universe.
For "cX" values, the algorithm is essentially the same, except that LAM will resolve "cX" to a specific node before spawning, and successive processes are spawned on the node where "cK" resides, where K = ((X + i) % total_cpus).
For example, if there are 8 nodes and 16 CPUs in the current LAM universe (2 CPUs per node), a "lam_spawn_sched_round_robin" key is given with the value of "c14", and maxprocs is 4, LAM will spawn MPI
CPU Node MPI_COMM_WORLD rank --- ---- ------------------- c14 n7 0 c15 n7 1 c0 n0 2 c1 n0 3
This key is used to designate that the spawned processes must not be spawned or scheduled on the "root node" (the node doing the spawn). There is no specific value associated with this key, but it should be given some non-null/non-empty dummy value.
It is a node-specific key and not a CPU-specific one. Hence if the root node has multiple CPUs, none of the CPUs on this root node will take part in the scheduling of the spawned processes.
No keys given
If none of the info keys listed above are 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 repeats.
The pre-defined attribute on MPI_COMM_WORLD , MPI_UNIVERSE_SIZE , can be useful in determining how many CPUs are currently unused. For example, the value in MPI_UNIVERSE_SIZE is the number of CPUs that LAM was booted with (see MPI_Init(1)). Subtracting the size of MPI_COMM_WORLD from this value returns the number of CPUs in the current LAM universe that the current application is not using (and are therefore likely not being used).
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 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 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.
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 .
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-mpi.org/contact.php ).
- - This error class is associated with an error code that indicates that free space is exhausted.
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), 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 formatting, and some initial text for most of the MPI-1 man pages.