[petsc-users] KSP convergence problem

[Jed Brown]

On Wed, Mar 20, 2013 at 3:13 PM, Mehrdad H Farahani
<mh.farahani at gmail.com>wrote:

> I am solving a Poisson equation with Neumann boundary conditions applied
> along an arbitrary curve using a ghost fluid treatment. The matrix is
> non-symmetric and the null space contains more than the constant vector. I
> have implemented a solve for the null space itself and have previously
> supplied it using MatSetNullSpace. Unfortunately the null space solve is
> extremely expensive and does not play well with GAMG or ML. We've checked
> that the answers we get without supplying this null space are very close to
> the answers we get when we do include it. The matrix for the two solves in
> the previous attachment are identical. The problem also seems to be
> converging and then suddenly diverges. Is this common behavior for a
> singular preconditioner?
>

If the RHS is not consistent or the preconditioner has a different null
space than the operator, you will have problems.
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