[petsc-users] fieldsplit question

[Sanjay Govindjee]

Thanks Mark.  I think I know what to do now.  Time to start coding.

-sanjay

On 4/4/25 2:17 AM, Mark Adams wrote:
> 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!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3Btpjtuwg$ >(ISDestroy 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3AhggVmlQ$ >(&field0));PetscCall 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/Sys/PetscCall/__;!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3Btpjtuwg$ >(ISDestroy 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3AhggVmlQ$ >(&field1));PetscCall 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/Sys/PetscCall/__;!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3Btpjtuwg$ >(ISDestroy 
> <https://urldefense.us/v3/__https://petsc.org/main/manualpages/IS/ISDestroy/__;!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3AhggVmlQ$ >(&field2));
>
>   ierr=PetscFree(idx1);
>
>   ierr=PetscFree(idx2);
>
>   fsSet=1;
>
> return0;
>
> }
>
>
>
> 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!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3C_POMvmw$ 
>     <https://urldefense.us/v3/__https://petsc.org/main/manualpages/PC/PCFieldSplitSetIS/__;!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57ydxKyWe$>
>     for each field. This is not very flexible, but the easiest to
>     understand.
>
>     2. Intermediate level
>
>     You can make a DMShell, and then make a PetscSection, that gives
>     the number of dofs on each vertex and edge. Then call KSPSetDM()
>     or SNESSetDM(), and you can do nested fieldsplits from the command
>     line. This also retains a connection between the topology and the
>     data layout, but you have to deal with that pesky DM object.
>
>     3. High level
>
>     You can use a DMPlex to represent your grid and a PetscFE to
>     represent the discretization, and then layout is done
>     automatically, and nested fieldsplits can be done from the command
>     line. I am not 100% sure PetscFE can represent what you want, but
>     you can always call DMPlexCreateSection() by hand to make the
>     PetscSection.
>
>       Thanks,
>
>           Matt
>
>         -sanjay
>
>
>
>     -- 
>     What most experimenters take for granted before they begin their
>     experiments is infinitely more interesting than any results to
>     which their experiments lead.
>     -- Norbert Wiener
>
>     https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!YYxJCjLuXQe2lb7pLGBk_jFm3tcpXjp7fMV9Z2SjL4wXwdnCNa3qKfT2WvEkGU5XtL60cuWHGdn1_3ALPfLgEw$ 
>     <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!aMROjbrPD3RYXMpO8mIii7q8eXZX1uN-6F6-g_jcNLLXGCgYPt2JEDkIIQCHs_vNBhhxsiwQJaz57-FUThx2$>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20250404/bf6e98e9/attachment-0001.html>