[petsc-users] Preconditioning of laplace equation with boomeramg

[Dave May]

What's happening with the solve on the coarse grid?
Are you using a direct solver?
Are you sure the null space is removed from the coarse grid operator as well?


On 1 February 2012 11:02, Thomas Witkowski
<Thomas.Witkowski at tu-dresden.de> wrote:
> I'm about to write a preconditioner for a 2x2 block matrix coming from the
> FEM discretization (2D, equidistance mesh) of a 4th order PDE. The (0,0)
> block is the discrete laplace with Neumann boundary conditions. I want to
> precondition this block by using some amg iterations. Just for tests, the
> preconditioner should solve in each step the neumann problem with some rhs
> using a gmres solver with hypre/boomeramg preconditioner up to a given
> tolerance. But in most iterations it ends up with DIVERGED_DTOL. What could
> be the reason for? Is AMG is linear preconditioner? The constant null space
> of the matrix is set via KSPSetNullSpace.
>
> Thomas