[petsc-users] Preconditioning of laplace equation with boomeramg

[Thomas Witkowski]

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