[petsc-users] PC FieldSplit - How to Count the Iterations

Sonia Pozzi sonia.pozzi at usi.ch
Thu Mar 15 05:31:09 CDT 2018

```Perfect,

Thank you a lot,

Sonia
Ph.D. Student
Group of Prof. Rolf Krause
Institute of Computational Science
Università della Svizzera italiana
Center for Computation Medicine in Cardiology
Via Giuseppe Buffi, 13
CH-6900 Lugano
Switzerland

Email: pozzis at usi.ch <mailto:pozzis at usi.ch>
Phone: +41 58 666 4972

> On 14 Mar 2018, at 16:00, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Wed, Mar 14, 2018 at 11:21 PM, Sonia Pozzi <sonia.pozzi at usi.ch <mailto:sonia.pozzi at usi.ch>> wrote:
> Dear Barry,
>
> thank you for the answer. That helped a lot. Just a second curiosity.
> I’m setting A00 to be solved with preonly+lu. I obtain the following
> ksp_0  KSPGetTotalIterations: 26
> ksp_1  KSPGetTotalIterations: 22
> Residual ksp_0: 0 Reason ksp_0: 4
> Solution ksp_0 :  Convergence in 1 iterations.
> Residual ksp_1: 5.23074e-14 Reason ksp_1: 2
> Solution ksp_1 :  Convergence in 11 iterations.
> Initial system linear residual:  9.904804e-09
>
> To me is clear that kps_0 iterations number is equal to 22*1+2*2.
> But why is the iterations number for the schur complement multiplied by two?
> I’m giving the mass matrix as a preconditioner for the schur complement.
>
> Possible cause:
>
>   The preconditioner is applied once before the solve happens in order to compute the preconditioned initial residual. Then
>   if you do just one iterate in the solve, you would have two applications of the preconditioner.
>
>   Thanks,
>
>     Matt
>
> Sonia
>
> Ph.D. Student
> Group of Prof. Rolf Krause
> Institute of Computational Science
> Università della Svizzera italiana
> Center for Computation Medicine in Cardiology
> Via Giuseppe Buffi, 13 <https://maps.google.com/?q=Via+Giuseppe+Buffi,+13+CH-6900+Lugano+Switzerland&entry=gmail&source=g>
> CH-6900 Lugano <https://maps.google.com/?q=Via+Giuseppe+Buffi,+13+CH-6900+Lugano+Switzerland&entry=gmail&source=g>
> Switzerland <https://maps.google.com/?q=Via+Giuseppe+Buffi,+13+CH-6900+Lugano+Switzerland&entry=gmail&source=g>
>
> Email: pozzis at usi.ch <mailto:pozzis at usi.ch>
> Phone: +41 58 666 4972 <tel:+41%2058%20666%2049%2072>
>> On 14 Mar 2018, at 14:52, Smith, Barry F. <bsmith at mcs.anl.gov <mailto:bsmith at mcs.anl.gov>> wrote:
>>
>> KSPGetIterationNumber
>
>
>
>
> --
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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