Starting New Tasks

• Basic MPI routine:
  MPI_Comm_spawn(char *command, char *argv[], int maxprocs, MPI_Info info, int root, MPI_Comm comm, MPI_Comm *intercomm, int errcodes[])
  starts new tasks
  returns intercommunicator between old and new tasks
  new tasks have their own independent MPI_COMM_WORLD
  arguments:

  • command	program name for new tasks method to find the executable is implementation-dependent
    argv	command line arguments of new tasks starts with first parameter (not with program name) last element: empty string (no argc !) MPI_ARGV_NULL predefined
    maxprocs	number of tasks to start
    info	system dependent information, some reserved keys: host (name of machine) path (place to look for executable) file (external file with infos) soft (list of alternative values for maxcount)
    root	task that evaluates this arguments
    comm	communicator of starting tasks
    intercomm	Intercommunicator between old and new tasks
    errorcodes	array of length maxprocs for errorcodes MPI_SUCCESS for success

• additional MPI routines:
  MPI_Comm_spawn_multiple

  • spawn with different command names und arguments for the new tasks
  • advantage to several spawns: all tasks use the same intercommunicator
  MPI_Comm_get_parent

  • intercommunicator from spawn for the new tasks
  MPI_UNIVERSE_SIZE

  • indicates maximal number of tasks
  • attribute of MPI_COMM_WORLD
  • can be obtained with MPI_Attr_get
• Example program (spawn.c, slave.c)
  master/slave version of PI program
  PVM-like model: one master task starts many slaves

  • call executable directly, not via mpirun
  • no advantage over MPI-1 model (except similarity to PVM)
  computes number of slaves from MPI_UNIVERSE_SIZE
  broadcast and reduce use intercommunicator!

  • slaves get connection to master with MPI_Comm_get_parent