[petsc-users] Termination criteria for solving linear system

[Jed Brown]

On Tue, 02 Feb 2010 13:41:23 +0100, Thomas Witkowski <thomas.witkowski at tu-dresden.de> wrote:
> Hallo Markus,
> 
> I just want to reduce the precision of the solution of the system of 
> linear equations! To do this it is not required (and in my simulations 
> also not possible) to reduce the number of degrees of freedom. It is a 
> question of how to define the solver parameters to cancel the solution 
> process at the right moment.

First, run with -ksp_monitor_true_residual

This shows both preconditioned and unpreconditioned residuals.  There
are two commonly used tolerances, relative and absolute.  If your
problem is well-scaled, the relative tolerance is the one that is most
likely to terminate the iteration.  Run with -ksp_converged_reason to
see why it converged.  With this information, you can decide which
tolerance to use (e.g. -ksp_rtol 1e-3 -ksp_atol 1e-6).  Also, some KSPs
support different norm types, you can set these with -ksp_norm_type.

Jed