[petsc-users] Termination criteria for solving linear system

Thomas Witkowski thomas.witkowski at tu-dresden.de
Wed Feb 3 06:01:49 CST 2010

Barry Smith wrote:
> On Feb 2, 2010, at 6:55 AM, Jed Brown wrote:
>> 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.
>    Specifically for the BiCG in PETSc you can use -ksp_norm_type 
> unpreconditioned and it will use the unpreconditioned residual norm 
> for the convergence test.
Thanks, thats exactly what I've searched for!

>    Barry
>> Jed

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