[petsc-users] fieldsplit question

[Mark Adams]

Matt, Matt, Matt,

Sanjay, (1) is the only sane option. Here is an example taken from a code
that I work with.

This simply has 3 fields with 12 ("stride", not a great name, should be
"bs") dofs on each vertex in each field (2D Bell + 1D cubic Hermite).
This uses ISCreateBlock, which you might want to use for dof (1,2) in your
case. It lets, with stride = 2, input IS = [1,3] create an IS with
[2,3,6,7] semantically.

Thanks,
Mark

int matrix_solve:: setFieldSplitType()

{

  // the global parameters

  PetscInt ierr, dofPerEnt,stride,k;

  int startDof, endDofPlusOne;

  int num_own_ent=m3dc1_mesh::instance()->num_own_ent[0], num_own_dof;

  m3dc1_field_getnumowndof(&fieldOrdering, &num_own_dof);

  if (num_own_ent) dofPerEnt = num_own_dof/num_own_ent;

  stride=dofPerEnt/3; //U 0->11, Omega 12->23, Chi 24->35

  m3dc1_field_getowndofid (&fieldOrdering, &startDof, &endDofPlusOne);

  startDof=startDof/stride;


  // the 3 fields for PCFIELDSPLIT

  IS field0, field1, field2;

  PetscInt *idx0, *idx1, *idx2;

  ierr=PetscMalloc1(num_own_ent, &idx0);

  ierr=PetscMalloc1(num_own_ent, &idx1);

  ierr=PetscMalloc1(num_own_ent, &idx2);


  for (k=0; k<num_own_ent; k++) idx0[k]=k*dofPerEnt/stride + startDof;

  ierr=ISCreateBlock(PETSC_COMM_WORLD, stride, num_own_ent, idx0,
PETSC_COPY_VALUES, &field0);


  for (k=0; k<num_own_ent; k++) idx1[k]=1+k*dofPerEnt/stride + startDof;

  ierr=ISCreateBlock(PETSC_COMM_WORLD, stride, num_own_ent, idx1,
PETSC_COPY_VALUES, &field1);


  for (k=0; k<num_own_ent; k++) idx2[k]=2+k*dofPerEnt/stride + startDof;

  ierr=ISCreateBlock(PETSC_COMM_WORLD, stride, num_own_ent, idx2,
PETSC_COPY_VALUES, &field2);


  PC pc;

  ierr= KSPAppendOptionsPrefix(*ksp,"fs_"); // ksp is a global here

  ierr=KSPGetPC(*ksp, &pc);

  ierr=PCSetType(pc, PCFIELDSPLIT);

  ierr=PCFieldSplitSetIS(pc, NULL, field0);

  ierr=PCFieldSplitSetIS(pc, NULL, field1);

  ierr=PCFieldSplitSetIS(pc, NULL, field2);


  ierr=PetscFree(idx0);PetscCall
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/Sys/PetscCall/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9gefqEq4$ >(ISDestroy
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9VDKwi_8$ >(&field0));PetscCall
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/Sys/PetscCall/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9gefqEq4$ >(ISDestroy
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9VDKwi_8$ >(&field1));PetscCall
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/Sys/PetscCall/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9gefqEq4$ >(ISDestroy
<https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9VDKwi_8$ >(&field2));

  ierr=PetscFree(idx1);

  ierr=PetscFree(idx2);

  fsSet=1;

  return 0;

}


On Thu, Apr 3, 2025 at 8:57 AM Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Apr 3, 2025 at 12:11 AM Sanjay Govindjee via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> We would like to solve an FEA problem (unstructured grid) where the nodes
>> on the elements have different dofs.  For example the corner nodes have
>> only dof 0 and then mid-side nodes have dofs 0,1,2  (think 8 node
>> serendipity element).  This is a multi-physics problem so we are looking to
>> use the fieldsplit features to pre-condition and solve.  Is there a simple
>> example of this type of usage in the src that we can try to mimic?
>>
>> I presume this will take programming as opposed to just setting command
>> line options.
>>
>
> It will take a little programming, but not much. Here is the idea.
> FieldSplit needs to know what dofs belong to what field. There are a couple
> of ways to do this, at different levels of abstraction.
>
> 1. Low level
>
> You can explicitly makes lists of the dofs in each field, as an IS, and
> call https://urldefense.us/v3/__https://petsc.org/main/manualpages/PC/PCFieldSplitSetIS/__;!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9-jY5Sfg$ 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/PC/PCFieldSplitSetIS/__;!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57ydxKyWe$>
> for each field. This is not very flexible, but the easiest to understand.
>
> 2. Intermediate level
>
> You can make a DMShell, and then make a PetscSection, that gives the
> number of dofs on each vertex and edge. Then call KSPSetDM() or
> SNESSetDM(), and you can do nested fieldsplits from the command line. This
> also retains a connection between the topology and the data layout, but you
> have to deal with that pesky DM object.
>
> 3. High level
>
> You can use a DMPlex to represent your grid and a PetscFE to represent the
> discretization, and then layout is done automatically, and nested
> fieldsplits can be done from the command line. I am not 100% sure PetscFE
> can represent what you want, but you can always call DMPlexCreateSection()
> by hand to make the PetscSection.
>
>   Thanks,
>
>       Matt
>
>
>> -sanjay
>>
>>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
> https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!c_nKrPbKpTddk11qYmEtY65UqhTbG4LXm7rfRLNAprFTA0x4y_NfBtBIEhOMant3sxPS91A_TgyDkqZ9zTSheMM$ 
> <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57-FUThx2$>
>
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