# Open MPI User's Mailing List Archives

 |   Home   |   Support   |   FAQ   |   all Open MPI User's mailing list
 Subject: [OMPI users] try to understand heat equation 2D mpi version From: christophe petit (christophe.petit09_at_[hidden]) Date: 2010-10-21 20:17:54 Hello, i'm studying the parallelized version of a solving 2D heat equation code in order to understand cartesian topology and the famous "MPI_CART_SHIFT". Here's my problem at this part of the code : ------------------------------------------------------------------------------------------------------------------------------------------- call MPI_INIT(infompi)       comm = MPI_COMM_WORLD       call MPI_COMM_SIZE(comm,nproc,infompi)       call MPI_COMM_RANK(comm,me,infompi) ! ...... ! Create 2D cartesian grid       periods(:) = .false.       ndims = 2       dims(1)=x_domains       dims(2)=y_domains       CALL MPI_CART_CREATE(MPI_COMM_WORLD, ndims, dims, periods, &         reorganisation,comm2d,infompi) ! ! Identify neighbors !       NeighBor(:) = MPI_PROC_NULL ! Left/West and right/Est neigbors       CALL MPI_CART_SHIFT(comm2d,0,1,NeighBor(W),NeighBor(E),infompi)       print *,'rank=', me       print *, 'here first mpi_cart_shift : neighbor(w)=',NeighBor(W)       print *, 'here first mpi_cart_shift : neighbor(e)=',NeighBor(E) ... ------------------------------------------------------------------------------------------------------------------------------------------- with x_domains=y_domains=2 and i get at the execution :" mpirun -np 4 ./explicitPar"  rank= 0 here first mpi_cart_shift : neighbor(w)= -1  here first mpi_cart_shift : neighbor(e)= 2 rank= 3  here first mpi_cart_shift : neighbor(w)= 1  here first mpi_cart_shift : neighbor(e)= -1  rank= 2  here first mpi_cart_shift : neighbor(w)= 0  here first mpi_cart_shift : neighbor(e)= -1  rank= 1  here first mpi_cart_shift : neighbor(w)= -1  here first mpi_cart_shift : neighbor(e)= 3 I saw that if the rank is out of the topology and wihtout periodicity, the rank should be equal to MPI_UNDEFINED whis is assigned to -32766 in "mpif.h" . So, why have i got the value "-1" ? On my Macbook pro, i get the value "-2". Thanks in advance. text/html attachment: attachment