From src/ksp/pc/impls/fieldsplit/fieldsplit.c this is how the saddle point is
detected and set into the PC
if (jac->detect) {
IS zerodiags,rest;
PetscInt nmin,nmax;
ierr = MatGetOwnershipRange(pc->mat,&nmin,&nmax);CHKERRQ(ierr);
if (jac->diag_use_amat) {
ierr = MatFindZeroDiagonals(pc->mat,&zerodiags);CHKERRQ(ierr);
} else {
ierr = MatFindZeroDiagonals(pc->pmat,&zerodiags);CHKERRQ(ierr);
}
ierr = ISComplement(zerodiags,nmin,nmax,&rest);CHKERRQ(ierr);
ierr = PCFieldSplitSetIS(pc,"0",rest);CHKERRQ(ierr);
ierr = PCFieldSplitSetIS(pc,"1",zerodiags);CHKERRQ(ierr);
ierr = ISDestroy(&zerodiags);CHKERRQ(ierr);
ierr = ISDestroy(&rest);CHKERRQ(ierr);
In addition these two options are set
PetscErrorCode PCFieldSplitSetDetectSaddlePoint(PC pc,PetscBool flg)
{
PC_FieldSplit *jac = (PC_FieldSplit*)pc->data;
PetscErrorCode ierr;
PetscFunctionBegin;
jac->detect = flg;
if (jac->detect) {
ierr = PCFieldSplitSetType(pc,PC_COMPOSITE_SCHUR);CHKERRQ(ierr);
ierr =
PCFieldSplitSetSchurPre(pc,PC_FIELDSPLIT_SCHUR_PRE_SELF,NULL);CHKERRQ(ierr);
}
PetscFunctionReturn(0);
}
You can use these routines to directly manage the IS yourself in any manner you
choice.
Good luck
Barry
> On Oct 18, 2020, at 2:55 PM, Namala, Solomon <[email protected]> wrote:
>
> Hello,
>
> I am working to solve Stokes problem on a domain that is discretized using
> two different types of mesh. A part of the mesh uses fem formulation and the
> rest uses nodal integral method (NIM) formulation (the details of which I
> will skip). However, the key takeaway is that NIM formulation of stokes uses
> pressure Poisson formulation instead of the continuity equation while FEM
> formulation uses the continuity equation. They are coupled at the interface.
> Right now, I am building a single matrix for the entire domain and solving it
> using fieldsplit option in a nested fashion. The matrix structure and the
> unknown vector are shown below.
>
> My questions are:
>
> Are there any basic guidelines to solve these kind of problems.
> As I have mentioned I am currently using nested fieldsplit. The first split
> is using indices and the other split is done using detect saddle point
> option. is there a way to avoid using that option and doing it by combining
> set of indices or fields.
> The matrix structure is
> [Au_fem Bp_fem 0 0]
> [Cu_fem. 0 0 0]
> [0 0 Du_nim Ep_nim]
> [0 0 0 Fp_nim]
>
> the unknown vector is given by
>
> [ufem pfem unim pnim]
>
> Let me know if any additional information is needed.
>
> Thanks,
> Solomon.
