3.9   Dynamic process groups

• Dynamic process group functions are build on the top of the core PVM routines
• Needs to link with a separate library: libgpvm3.a
• The pvmd does not perform the group functions
• A group server handles these functions; it is automatically started when a group function is first invoked
• New feature in PVM (since version 3.0) ® experimental, feedback is encouraged
• Debate:
  • Static or dynamic grouping
  • Only tasks in a group can call group functions
  • Efficiency/reliability
• Any PVM task may
  • Join or leave any group at any time
  • Without having to inform any other task
• Tasks can broadcast messages to groups of which they are not a member
  
  
• Broadcast message are not send back to the sender

• int inum = pvm_joingroup (char *group)
call pvmfjoingroup (group, inum) int info = pvm_lvgroup (char *group)
call pvmflvgroup (group, info)
  • Allows a task to join a user name group
  • The first call to pvm_joingroup() creates a group with name group
  • A task can join multiple group
  • pvm_joingroup() returns the instance number (inum) of the process in this group
  • Instance numbers run from 0 to the number of group member minus 1
  • Instance numbers are recycled so a task joining a group will get the lowest available instance number
  • So there is no guarantee that the instance numbers are in a continuous set

• int tid = pvm_gettid (char *group, int inum)
call pvmfgettid (group, inum, tid)
  • Returns the tid of the process with a given group name and instance number
• int inum = pvm_getinst (char *group, int tid)
call pvmfgetinst (group, tid, inum)
  • Returns the instance number of the task tid in the specified group
• int size = pvm_gsize (char *group)
  call pvmfgsize (group, size)|
  • Returns the number of members in the specified group

• int info = pvm_barrier (char *group, int count)
call pvmfbarrier (group, count, info)
  • The calling process blocks until count member of the group have call pvm_barrier()
  • In general, count should be the total number of members of the group
  • The logical function of the routine is to provide a group synchronization
  • count is required because of the dynamic aspect of PVM groups
  • The calling process must be a member of the group
  • All the calling processes must set the same count
  • If count is set to -1, PVM uses the value of pvm_gsize(). Useful if the group is not changing.
  • Example

    inum = pvm_joingroup ("worker") ; 
    ...
    info = pvm_barrier ("worker", pvm_gsize ("worker")) ;
              

• int info = pvm_bcast (char *group, int msgtag)
call pvm_bcast (group, msgtag, info)
  • Labels the message with the identifier msgtag
  • Broadcasts the message to all the tasks in the specified group
  • ``All the tasks'' is defined to be those tasks the group server thinks are in the group when the routine is called
  • Example

    info = pvm_initsend (PvmDataRaw) ; 
    info = pvm_pkint (array, 10, 1) ; 
    msgtag = 5 ; 
    pvm_bcast ("worker", msgtag) ;