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Dynamic Process Groups

Dynamic Process Groups

• Group functions are built on top of the core PVM routines

• libgpvm3.a must be linked with user programs using any of the group functions

• pvmd does not perform the group functions - they are performed by a group server which is automatically started when the first group function is invoked.

• Any PVM task can join or leave a group at any time without having to inform any other task in the affected groups.

• Tasks can broadcast messages to groups of which they are not a member

• Any PVM task may call any of the group functions except for - pvmflvgroup(), pvmfbarrier(), and pvmfreduce()

Joining a Group

pvmfjoingroup

• pvmfjoingroup creates a group -

• puts the calling task in the group

• group is a character string containing the group name

• returns the instance number, inum, of the process in this group - ie a number from 0 to the number of group members -1

• a task may join multiple groups

• on leaving and rejoining a group a task may receive a different instance number

• instance numbers are recycled and a task joining a group gets the lowest instance number available

Leaving a Group

pvmflvgroup

• pvmflvgroup() removes a task from a group but does not return until the task is confirmed to have left, a subsequent pvmfjoingroup will assign the vacant instance number to the new task

CALL PVMFLVGROUP(GROUP, INFO)

General Group Functions

• The following functions may be used to perform group related actions
  • pvmfgettid

  • pvmfgetinst

  • pvmfgsize

  • pvmfbarrier

  • pvmfbcast

  • pvmfreduce

Group Functions

• pvmfgettid() returns the TID of the process with a given group name and instance number, it also allows two tasks to get each other's TID simply by joining a common group

• pvmfgetinst returns the instance number of TID in the specified group

• pvmfgetsize returns the number of members in the group

• pvmfbarrier() blocks a process until count number of processes have joined the group.

• pvmfbcast broadcasts to all the tasks in the specified group except itself ie all tasks that the group server thinks are in the group when the routine is called

• pvmfreduce() performs a global arithmetic operation across the group, the result appears on root eg predefined functions that the user can place in func - PvmMax, PvmMin, PvmProduct, and PvmSum

Example SPMD

c ie me=0 spawn more copies of myself

call pvmfjoingroup(`foo', me)

if(me .eq. 0) call pvmfspawn (`spmd',

& PVMDEFAULT, '*', NPROC-1, tids(1), info)

c Wait for everyone to startup before proceeding.

call pvmfbarrier(`foo', NPROC, info)

if(me .eq. 0) print*, 'setup complete'

call dowork(me, NPROC)

c program finished leave group and exit pvm

call pvmflvgroup(`foo', info)

... etc

subroutine dowork(me, nproc)

include `/totem/pvm3/include/fpvm3.h'

c Simple subroutine to pass a token around a ring

integer me, nproc

integer token, src, dest, msgtag

c Determine neighbours in the ring

call pvmfgettid(`foo', me-1, src)

call pvmfgettid(`foo', me+1, dest)

if(me .eq. 0) then

call pvmfgettid(`foo', NPROC-1,src)

else if(me .eq. NPROC-1) then

call pvmfgettid(`foo', 0, dest)

endif

msgtag = 4

call pvmfinitsend(PVMDEFAULT, info)

if (me .eq. 0) then

token = dest

call pvmfpack(INTEGER4, token, 1, 1, info)

call pvmfsend(dest, msgtag, info)

call pvmfrecv(src, msgtag, info)

print*, `token ring done'

else

call pvmfrecv(src, msgtag, info)

call pvmfunpack(INTEGER4, token,1,1, info)

token = dest

call pvmfpack(INTEGER4, token, 1, 1, info)

call pvmfsend(dest, msgtag, info)

endif