[petsc-users] fieldsplit: Unhandled case, must have at least two fields, not 1!

[Jed Brown]

Nico Schlömer <nico.schloemer at gmail.com> writes:

> Hi all,
>
> I'm toying around with PETSc's field split capabilities for the
> 2D-Stokes problem, and similar to ex42 I call
>
>   // set of the operators
>   // set of a bunch of options
>   PetscOptionsSetValue("-pc_type", "fieldsplit");
>   PC pc;
>   ierr = KSPGetPC(*_ksp, &pc);
>   const PetscInt ufields[] = {0,1}, pfields[] = {2};
>   ierr = PCFieldSplitSetBlockSize(pc,3);
>   PCFieldSplitSetFields(pc,"u",2,ufields,ufields);
>   PCFieldSplitSetFields(pc,"p",1,pfields,pfields);
>
> When finally solving, however, I'm getting the error message
>
> [0]PETSC ERROR: Petsc has generated inconsistent data!
> [0]PETSC ERROR: Unhandled case, must have at least two fields, not 1!

Short answer: use PCFieldSplitSetIS().  From the man page:

    Notes: Use PCFieldSplitSetIS() to set a completely general set of indices as a field.

     The PCFieldSplitSetFields() is for defining fields as strided blocks. For example, if the block
     size is three then one can define a field as 0, or 1 or 2 or 0,1 or 0,2 or 1,2 which mean
     0xx3xx6xx9xx12 ... x1xx4xx7xx ... xx2xx5xx8xx.. 01x34x67x... 0x1x3x5x7.. x12x45x78x....
     where the numbered entries indicate what is in the field.

     This function is called once per split (it creates a new split each time).  Solve options
     for this split will be available under the prefix -fieldsplit_SPLITNAME_.

     Developer Note: This routine does not actually create the IS representing the split, that is delayed
     until PCSetUp_FieldSplit(), because information about the vector/matrix layouts may not be
     available when this routine is called.
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