Open MPI User's Mailing List Archives

Subject: Re: [OMPI users] try to understand heat equation 2D mpi version
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-10-22 02:00:50

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christophe petit wrote:

> 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".

It seems to me the man page says MPI_PROC_NULL may be returned, and in
OMPI that looks like -2. Can you try the following:

% cat x.f90
  include "mpif.h"

  integer comm, dims(2)
  logical periods(2)

  call MPI_INIT(ier)
  ndims = 2
  dims(1)=2; periods(1) = .false.
  dims(2)=2; periods(2) = .false.
  CALL MPI_CART_CREATE(MPI_COMM_WORLD, ndims, dims, periods, .false.,
comm, ier)
  CALL MPI_CART_SHIFT(comm, 0, 1, iwest, ieast, ier)
  write(6,*) iwest, ieast, MPI_PROC_NULL
  call MPI_Finalize(ier)
end
% mpif90 x.f90
% mpirun -n 4 ./a.out
 1 -2 -2
 -2 2 -2
 -2 3 -2
 0 -2 -2

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