use mpi_cart_create with master and slave - fortran

Question

I want to create a topology without 0 processor in it. My idea is to make processor 0 as master and create topology as slave. After certain computation sin topology, I will send data to master. Here is my code:
include "mpif.h"
integer maxn
integer myid,Root,numprocs,numtasks,taskid
integer comm2d, ierr
integer dims(2)
logical periods(2)
data periods/2*.false./
Root = 0
CALL MPI_INIT( ierr )
CALL MPI_COMM_RANK(MPI_COMM_WORLD, myid, ierr )
CALL MPI_COMM_SIZE(MPI_COMM_WORLD, numprocs, ierr )
numtasks = numprocs-1
if(myid .eq. Root) then
print *, 'Hello I am master'
endif
c Get a new communicator for a decomposition of the domain.
c Let MPI find a "good" decomposition
dims(1) = 0
dims(2) = 0
CALL MPI_DIMS_CREATE(numtasks,2,dims,ierr)
CALL MPI_CART_CREATE(MPI_COMM_WORLD,2,dims,periods,.true.,
* comm2d,ierr)
c Get my position in this communicator
CALL MPI_COMM_RANK( comm2d, taskid, ierr )
c
print *, 'task ID= ',taskid
if (myid .eq. master) then
print *,dims(1),dims(2)
endif
CALL MPI_Comm_free( comm2d, ierr )
30 CALL MPI_FINALIZE(ierr)
STOP
END
But, when I run above code; I am getting the following error.
Fatal error in PMPI_Comm_rank: Invalid communicator, error stack:
PMPI_Comm_rank(121): MPI_Comm_rank(MPI_COMM_NULL, rank=0x7fff08bf960c)
failed PMPI_Comm_rank(73).: Null communicator
How can I eliminate the error? What I am doing wrong.

Answer 1

You start your MPI job with numprocs processes. Then you create a Cartesian topology with numtasks = numprocs-1 processes. Therefore, one of the processes ends up not being part of the Cartesian communicator and receives MPI_COMM_NULL in comm2d. Calling MPI_COMM_RANK with a null communicator is an error. The solution is to fix your code to first check the value of comm2d:
CALL MPI_CART_CREATE(MPI_COMM_WORLD,2,dims,periods,.true.,
* comm2d,ierr)
IF (comm2d.NE.MPI_COMM_NULL) THEN
...
ENDIF