Extension of operations to intercommunicators

• MPI-2 extends the meaning of some (mainly collective) functions to intercommunicators.
  
• Creation of intercommunicators
  MPI_Comm_create(MPI_Comm old, MPI_Group group, MPI_Comm *new)
  meaning: newcomm contains the corresponding local subgroups

  • (the group argument has to be identical inside a local group)
  
• unidirectional data flow (functions with root task)
  MPI_Bcast(void* buf, int count, MPI_Datatype type, int root, MPI_Comm comm)
  Meaning: task root in group A sends to all tasks in group B
  

       root = MPI_ROOT      for task root,
            = MPI_PROC_NULL for other tasks of group A
            = rank(root)    for all tasks of group B
  

• bidirectional data flow (functions without root task)
  MPI_Allreduce(void* sendbuf, void* recvbuf, int count, MPI_Datatype type, MPI_Op op, MPI_Comm comm)
  Meaning: all tasks in group B receive the reduction of all values in group A and vice versa.