python - mpi4py: Communicating between spawned processes -

i have 1 process running program called t1.py spawns 3 other processes, of run t2.py. want broadcast value spawned process rank of 0 2 other spawned processes. however, when bcast called, program blocks. idea why happens? , how fix it?

t1.py

from mpi4py import mpi import sys  sub_comm = mpi.comm_self.spawn(sys.executable, args=['t2.py'], maxprocs=3) print 'hi' 

t2.py

from mpi4py import mpi  comm = mpi.comm.get_parent()  print 'ho ', comm.get_rank() = comm.bcast(comm.get_rank(), root=0) print 

output

hi ho  2 ho  0 ho  1 

if want childs talk each other, can use mpi.comm_world:

a = mpi.comm_world.bcast(mpi.comm_world.get_rank(), root=0) 

by printing mpi.comm_world.get_rank(), ' of ',mpi.comm_world.get_size(), can check childs'mpi.comm_world limited childs.

now, let's investigate reason why comm.bcast(...) failed if comm obtained comm=mpi.comm.get_parent(). indeed, looking @ size , ranks of communicator, seems similar mpi.comm_world. but, on contrary, comm different mpi.comm_world: intercommunicator. more precisely, way parent can talk childs. collective communications can used, processes, both parent , childs, must call function. please carrefully read mpi standards, in particular sections 5.2.2 , 5.2.3 intercommunicator collective operations. regarding bcast(), mpi.root , mpi.proc_null used instead of rank of broadcaster root specify direction (parent child of child parent) , sending process. lastly, intracommunicator can defined on base of intercommunicator using merge() (corresponding mpi_intercomm_merge()). in intracommunicator, parents , childs not belong 2 different groups: processes characterized unique rank, usual.

here modified versions of t1.py , t2.py, bcast() intercommunicator performed. intercommunicator merge() , bcast() on resulting intracommunicator called usual.

t1.py

from mpi4py import mpi import sys  sub_comm = mpi.comm_self.spawn(sys.executable, args=['t2.py'], maxprocs=3)  val=42 sub_comm.bcast(val, mpi.root)  common_comm=sub_comm.merge(false) print 'parent in common_comm ', common_comm.get_rank(), ' of  ',common_comm.get_size() #mpi_intercomm_merge(parentcomm,1,&intracomm);  val=13 c=common_comm.bcast(val, root=0) print "value rank 0 in common_comm", c 

t2.py

from mpi4py import mpi  comm = mpi.comm.get_parent()  print 'ho ', comm.get_rank(), ' of  ',comm.get_size(),' ', mpi.comm_world.get_rank(), ' of  ',mpi.comm_world.get_size() = mpi.comm_world.bcast(mpi.comm_world.get_rank(), root=0) print "value other child",  print "comm.is_inter", comm.is_inter() b = comm.bcast(comm.get_rank(), root=0) print "value parent", b  common_comm=comm.merge(true) print "common_comm.is_inter", common_comm.is_inter() print 'common_comm ', common_comm.get_rank(), ' of  ',common_comm.get_size()  c=common_comm.bcast(0, root=0) print "value rank 0 in common_comm", c