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
