3.8   Communication realization

• int oldval = pvm_setopt (int what, int val)
call pvmfsetopt (what, val, oldval)
  
  
  • Sets options of the PVM system
  • what = PvmRoute advises PVM to set up direct task-to-task links for all subsequent communications or not
  • By default, PVM routes messages via daemons (UDP protocol)
  • Direct task-to-task communication are possible (using TCP)
    
    
    
  • val specifies one of

    PvmDontRoute

    Doesn't allow direct links to this task

    PvmAllowDirect

    Allows but doesn't requests direct links (It is the default)

    PvmRouteDirect

    Requests direct links

    	Route	Dont	Allow
    Route	direct	---	direct
    Dont	---	---	---
    Allow	direct	---	---

    
    

•    
  • Once a direct route established between tasks, both will use the route
  • Once a link is established, it remains until the application finishes
  • If a link can not be established
    • Because one of the two tasks has requested PvmDontRoute
    • Or because no resources are available
    The default route though the PVM daemons is used
  • pvm_setopt (PvmRoute...) can be called multiple times to selectively establish direct links
  • But it is typically set only once near the beginning of each task
  • Performance of direct task-to-task is better
  • Draw backs are
    • Lack of scalability
    • Startup time