[petsc-dev] Using PCFieldSplitSetIS

[Jed Brown]

On Wed, Mar 16, 2011 at 07:37, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> Thanks for explanations! It works fine in my code. But I have two questions
> about it, maybe you can help me with them:
> - To the first, is the LU factorization on block A_00 done only once?
>

Yes, one way to confirm this is to run with -log_summary and check
MatLUFactor{Sym,Num}.


> - I have run the code with -fieldsplit_1_ksp_monitor to get some more
> information about the internal solves. I expected to get information about
> one iterative solver (for solving the Schur complement system), but I got
> three, all of them need around 20 iterations for my example. Could you
> explain to me what is actually solved there?
>

I'm going to guess that you still have an outer KSP that (in the global
norm, rather than the partitioned norm used inside of splits) has a tighter
tolerance, therefore it takes a few outer iterations. If you use loose inner
tolerances then the preconditioner becomes nonlinear and you'll need to use
FGMRES for the outer. When in doubt, run with -ksp_view and show us the
results if you don't understand. Additionally, monitoring inner solves
separately can be useful, e.g. -fieldsplit_0_ksp_converged_reason
-fieldsplit_1_ksp_monitor -ksp_monitor_true_residual.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20110316/df39c162/attachment.html>