Dave, thank you for the hint. Yes, direct solver was set on coarse space. I changed it to Jacobi and it works as expected!
Thomas Am 01.02.2012 12:32, schrieb 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
