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

[Bin Liu]

Dear Barry,

Thanks for your explanation. Does it mean the default director solver LU in PETSc can also run in the distributed memory mode? I only used iterative solvers with preconditioners in distributed memory before.  My experience in using director solver in parallel is indeed limited, especially the saddle point problem in parallel. I have done some search online, but I did not find a working set up at this moment. Could you give a sample setup of the direct solver for parallel run? It is really appreciated.

From: Barry Smith [mailto:bsmith at petsc.dev]
Sent: Wednesday, 29 July 2020 9:48 AM
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


   SuperLU_DIST won't "magically" run on saddle point problems. It only does limited pivoting, realistically a parallel LU cannot always do complete pivoting or it becomes a sequential algorithm. For parallel you need to use PCFIELDSPLIT, for sequential you can use SuperLU (not SuperLU_DIST) since it can do more pivoting, being a sequential algorithm.

  Barry



On Jul 28, 2020, at 7:59 PM, Bin Liu <lbllm2018 at hotmail.com<mailto: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. 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<mailto:lbllm2018 at hotmail.com>>
Cc: petsc-users at mcs.anl.gov<mailto: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<mailto: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

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