NULL Communicator Error, Invalid Communicator

[julym...@gmail.com]

Hello,

I am just now starting to work with mpi4py and I am running into the following error: 

mpi4py.MPI.Exception: Invalid communicator, error stack: 

PMPI_Comm_rank(109): MPI_Comm_rank(MPI_COMM_NULL, rank=0x7fffffff5d5c) failed 

PMPI_Comm_rank(66).: Null communicator

I have a python function defined in a file that is spawning worker processes:

---------------------------------

def search_direction(f, graph, g, od):

    x = np.power(f.reshape((f.shape[0],1)), np.array([0,1,2,3,4]))

    grad = np.einsum('ij,ij->i', x, graph[:,3:])

    g.es["weight"] = grad.tolist()

    

    L = np.zeros(len(g.es),dtype='float64')

    #The following code is for MPI

    comm = MPI.COMM_SELF.Spawn(sys.executable,

                           args=['AoN_igraph_MPI.py'],

                           maxprocs=2)

    comm.bcast(od, root=MPI.ROOT)

    comm.bcast(g, root =MPI.ROOT)

    comm.Reduce(none, L, op=MPI.SUM, root= MPI.ROOT)

    return L, grad

------------------------------------

The work/child code is as follows (saved in AoN_igraph_MPI.py):

--------------------------------------------------

from mpi4py import MPI

from mpi4py.MPI import ANY_SOURCE

comm = MPI.Comm.Get_parent()

rank = comm.Get_rank()  #Rank of the particular process

size = comm.Get_size()  #number of processes

 ..... 

---------------------------------------

Now, the code executes past the "comm = MPI.Comm.Get_parent()" command, but then stops with error whenever I try to get the rank or size in the work process. Is this a problem with the fact that it's a function that is spawning the worker processes? The search_direction function is called by another function solver_3, residing in the same file, which in turn is called by the main function in the file. Does mpi4py allow for function definition?

I should also mention that I have successfully ran the "Compute Pi" example provided in the tutorial, so I don't think it it a problem with the mpi4py installation on my machine. Also, this all running on a Linux machine.

Help with this would be greatly appreciated.

Juliette

[Lisandro Dalcin]

Have you tried using another example using spawn? Any chances that you have multiple installs of mpi4py? If you spawn a child, and in that child Comm.Get_parent() returns the null communicator, well, either your MPI is not working properly, or the MPI being used in the child is not the same as the one in the parent process. This would happen if you have multiple mpi4py installs with different MPI backends, and somehow (PYTHONPATH environ var?) the parent imports one of the mpi4py's, but the child is importing the other.

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Lisandro Dalcin
============
Research Scientist
Computer, Electrical and Mathematical Sciences & Engineering (CEMSE)
Extreme Computing Research Center (ECRC)
King Abdullah University of Science and Technology (KAUST)
http://ecrc.kaust.edu.sa/

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