[petsc-users] Fieldsplit for 4 fields

[Matthew Knepley]

On Fri, Mar 11, 2022 at 3:32 PM Tang, Qi <tangqi at msu.edu> wrote:

> Hi,
> I am trying to solve a four field system with three nested fieldsplit (I
> am in petsc/dmstag directly). I think I have all the IS info in the
> original system. I am wondering how to set up IS for the split system.
>

Some questions first:

  1) Are you using DMStag? If so, the field split might be able to be
automated.

  2) Checking the division:

       Schur complement {1, 2, 3} and {0}
          Multiplicative {1,2} and {3}
              Schur complement {1} and {2}

    Thanks,

       Matt


> More specifically, I would like to call something like this
> -pc_type fieldsplit -pc_fieldsplit_type schur -pc_fieldsplit_0_fields 0
> -pc_fieldsplit_1_fields 1,2,3
>    -fieldsplit_1_ksp_type fgmres -fieldsplit_1_pc_type fieldsplit
> -fieldsplit_1_pc_fieldsplit_type multiplicative
>       -fieldsplit_1_fieldsplit_1_ksp_type fgmres
> -fieldsplit_1_fieldsplit_1_pc_type fieldsplit
> -fieldsplit_1_fieldsplit_1_pc_fieldsplit_type schur
>
> I know the first level probably would work. But the second and third
> levels would not.
>
> We have two components living on one type of dofs. So the natural split
> 0,1,2,3 do not work. Therefore, at the first level I am setting up split
> through
> PCFieldSplitSetIS(pc, “i", myis[i]);
> How could I know the sub ISs and set it up correctly? Thanks.
>
>
> Qi Tang
> T5 at LANL
>
>
>
>

-- 
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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