Use either the results for DMDAGetGhostCorners() or DMDAGetCorners() to
determine if the first or last node is local. This extends trivially to 2 and
3d. See for example src/ksp/ksp/examples/tutorials/ex34.c ComputeMatrix() if
(i==0 || j==0 || k==0 || i==mx-1 || j==my-1 || k==mz-1) { etc
Barry
> On Mar 18, 2015, at 9:06 PM, Adrian Croucher <[email protected]>
> wrote:
>
> hi,
>
> I'm setting up a simple 1-D finite difference test problem with Dirichlet
> boundary conditions at each end, using DMDA.
>
> I used DMDACreate1d() with the DM_BOUNDARY_GHOSTED option, and set the global
> array dimension to be the total number of nodes (including boundary nodes)
> minus 2- so the boundary nodes aren't included in the mesh.
>
> When I'm evaluating the differential equation on this mesh, I use
> DMGetLocalVector() and DMGlobalToLocalBegin/End() to scatter to my ghosted
> vector, then DMDAVecGetArray() to get an array on the local vector.
>
> When I want to apply the BCs at each end, I call DMDAGetGhostCorners() to
> find the indices of the ghost points. I can now plug the required boundary
> values into my array.
>
> At present, if I'm running on multiple processors, I'm just checking if
> rank=0 or rank=size-1 to make sure the BCs only get applied at the actual
> ends of the mesh, and not at the ends of the internal partitions as well.
>
> This works, but is there a better way to do it? It seems potentially
> unreliable- as it assumes rank 0 is always the left hand end and rank size-1
> always the right-hand end. Also it won't really work in 2D or 3D.
>
> Thanks!
>
> - Adrian
>
> --
> Dr Adrian Croucher
> Senior Research Fellow
> Department of Engineering Science
> University of Auckland, New Zealand
> email: [email protected]
> tel: +64 (0)9 923 84611
>
