[petsc-users] DMDA stencil for union jack triangular meshes

[Jed Brown]

On Tue, Jan 10, 2012 at 03:45, Thomas Witkowski <
Thomas.Witkowski at tu-dresden.de> wrote:

> I'm about to implement the multigrid preconditioner for my FEM code for
> the case of regulare 2D triangular meshes. My meshes have a union jack
> pattern, thus the nodes are equidistributed in both directions but the
> stencil varies between a 9-point and a 5-point stencil. Is it possible to
> fit this into the DMDA framework?


It's not easy to make DMCreateMatrix() preallocate for this arrangement,
but you can overallocate (all 9-point), use

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMSetMatrixPreallocateOnly.html

and then assemble what you have. The resulting matrix will be compacted so
that the used part is contiguous, so the run-time performance will be the
same as with perfect preallocation (but needed to allocate slightly more
memory).
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