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

[Jed Brown]

Mark Adams <mfadams at lbl.gov> writes:

>> [0]PETSC ERROR: Petsc has generated inconsistent data!
>> [0]PETSC ERROR: Unhandled case, must have at least two fields, not 1
>
>
> This can happen if you give fieldsplit options to a non-fieldsplit
> matrix/PC. So so calling XXXSetFromOptions with a KSP/PC and an AIJ Mat
> (ie, non-nested Mat)  and using a parameter like '-pc_fieldsplit_type
> schur', will give you this error.

It should not error.  FieldSplit existed years before MatNest and there
are lots of example that use it without MatNest (which is merely an
optimization, sometimes).

If you are getting this error, we need a complete stack trace and
instructions for how to reproduce (ideally using a PETSc example).
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