[petsc-users] petsc4py: reuse setup for multiple solver calls?

[Matthew Knepley]

On Thu, Apr 5, 2018 at 6:39 AM, Robert Speck <r.speck at fz-juelich.de> wrote:

> Hi!
>
> I would like to use petsc4py for my own Python library. Installation
> went well, first tests (serial and parallel) look good.
>
> Here is what I want to do: I have my own time-stepping playground and I
> want petsc4py to be one of my backbones for the data types and (linear
> or non-linear, serial or parallel) solvers. I don't want to use PETSc's
> time-steppers, at least not for now. So, I saw in some examples, in
> particular the ones shipped with petsc4py, that the standard way of
> running one of PETSc's solvers is a bunch of setup routines, then
> setting the right-hand side and solve.
>
> Now, I don't want to rerun the whole setup part each time I call the
> solver. I know that I can change the right-hand side without having to
> do so, but what if I change a parameter of my operator like, say, the
> time-step size or some material parameter?
>
> Take the simplest case: Say I have my own implicit Euler written in
> Python. I know the right-hand side F of my ODE, so in each step I want
> to solve "I - dt*F". But the time-step changes every now and then, so I
> cannot pre-assemble everything once and for all (or I don't want to).
> What do I need to rerun before I can use the solver again, what can I
> reuse? Could I just assemble F and combine it with the identity and the
> parameter dt right before I call the solver? How would that look like?
>
> I'm pretty new to PETSc and to petsc4py, so please forgive any stupidity
> or ignorance in these questions. I'm happy to take any advice, links to
> examples or previous questions. Thanks!
>

For linear solves which stay the same size, you just have to call
SetOperators
again with the new operator.

For nonlinear solves which stay the same size, you do nothing.

If the system size changes, it generally better to create the object. If
that is hard,
then you can use KSPReset().

   Matt


> Kind regards
> -Robert-
>
> --
> Dr. Robert Speck
> Juelich Supercomputing Centre
> Institute for Advanced Simulation
> Forschungszentrum Juelich GmbH
> 52425 Juelich, Germany
>
> Tel: +49 2461 61 1644
> Fax: +49 2461 61 6656
>
> Email:   r.speck at fz-juelich.de
> Website: http://www.fz-juelich.de/ias/jsc/speck_r
> PinT:    http://www.fz-juelich.de/ias/jsc/pint
>
>
>
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-- 
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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