[petsc-users] solving saddle point problem in distributed memory

[Matthew Knepley]

On Tue, Jul 28, 2020 at 9:00 PM Bin Liu <lbllm2018 at hotmail.com> wrote:

> Thanks for your tutorials. Yes. I tried PCFIELDSPLIT. However, it only
> works for sequential runs. When I run the code in distributed memory, it
> reports errors.
>

This would mean that your setup is incorrect. We run FIELDSPLIT all the
time in parallel, and it has been used on
some of the largest run ever made on supercomputers.

Second, the way to directly solve a saddle-point problem is to directly
solve the (0,0) block and the Schur complement. This is not hard:

  -pc_type fieldsplit
  -pc_fieldsplit_detect_saddlepoint
  -pc_fieldsplit_type schur
  -pc_fieldsplit_schur_fact_type full
  -pc_fieldsplit_schur_precondition full
  -fieldsplit_0_pc_type lu
  -fieldsplit_0_pc_factor_mat_solver_type superlu_dist
  -fieldsplit_1_pc_type lu
  -fieldsplit_1_pc_factor_mat_solver_type superlu_dist

  Thanks,

     Matt


> In fact, the essence of my wonder is (a) how to set up superlu_dist in
> petsc for solving saddle point problem in distributed memory? (b) does the
> direct solvers in petsc can run in distributed memory for solving saddle
> point problem?
>
>
>
> *From:* Stefano Zampini [mailto:stefano.zampini at gmail.com]
> *Sent:* Tuesday, 28 July 2020 6:55 PM
> *To:* Bin Liu <lbllm2018 at hotmail.com>
> *Cc:* petsc-users at mcs.anl.gov
> *Subject:* Re: [petsc-users] solving saddle point problem in distributed
> memory
>
>
>
> If you want advice you should post the error trace PETSc reports.
>
>
>
> Anyway, solving Stokes is not so trivial (without direct solvers, you may
> need mesh dependent information), but we have examples for it
>
>
>
> https://gitlab.com/petsc/petsc/-/blob/master/src/ksp/ksp/tutorials/ex42.c
>
> https://gitlab.com/petsc/petsc/-/blob/master/src/ksp/ksp/tutorials/ex43.c
>
> https://gitlab.com/petsc/petsc/-/blob/master/src/snes/tutorials/ex69.c
>
>
>
> If you scroll to the end of those files, you see a bunch of possible
> options either using PCFIELDSPLIT,  PCBDDC or KSPFETIDP.
>
>
>
>
>
> On Jul 28, 2020, at 12:37 PM, Bin Liu <lbllm2018 at hotmail.com> wrote:
>
>
>
> I would like to solve a saddle point problem arising from the stokes
> equation. I got successful to use the direct solvers in sequential runs.
> However, I would like to extend it for distributed memory computation. I
> tried to use superlu_dist, but the program returns errors. Is it possible
> to solve a saddle point problem in distributed memory using superlu_dist?
> Could anyone give a simple sample code to set up the parameters of the
> solver?
>
>
>
> Thanks
>
>
>


-- 
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.cse.buffalo.edu/~knepley/>
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