[petsc-users] Confusing Schur preconditioner behaviour

[Cotter, Colin J]

Hi Dave,

>If you are doing that, then you need to tell fieldsplit to use the Amat to define the splits otherwise it will define the Schur compliment as
>S = B22 - B21 inv(B11) B12
>preconditiones with B22, where as what you want is
>S = A22 - A21 inv(A11) A12
>preconditioned with B22.

>If your operators are set up this way and you didn't indicate to use Amat to define S this would definitely explain why preonly works but iterating on Schur does not.

Yes, thanks - this solves it! I need pc_use_amat.

all the best
--Colin
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