[petsc-dev] Iteration counter depends on KSP monitor!

Jed Brown jedbrown at mcs.anl.gov
Thu Nov 8 11:14:08 CST 2012

If you set a null space, you should make the right hand side consistent.

Have you checked the _accuracy_ of the solution in the case where a null
space was not used? When the system is singular, the error can be large
even with a small residual, but presumably the rest of your algorithm wants
a minimum norm solution.

On Thu, Nov 8, 2012 at 2:48 AM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> Maybe the following information is also useful to find the problem or/and
> to understand what's going on: The inner solver solves with a Laplace
> matrix (discretized with free boundaries). So I set the constant nullspace
> to the KSP object. This inner solver is used inside the Schur complement
> solver of a PCFIELDSPLIT object. The Schur complement solver (KSPPREONLY
> and PCSHELL) is also set to have a constant null space. But the vector, on
> which the PCSHELL is applied, is not orthogonal to the constant null space.
> Can you help me to understand why this is still the case?
> To go back to the problem of the influence of the KSP monitor to the
> solution process: When I project out the constant null space before calling
> the KSPRICHARDSON with PCHYPRE, the monitor has no influence anymore on the
> solution process.
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