[petsc-dev] [petsc-users] Shell preconditioner within a fieldsplit

[Jed Brown]

Patrick Sanan <patrick.sanan at gmail.com> writes:

> PCMAT seems abstraction-breaking (and the man page even admits it is
> "strange" :D )
>
> The preconditioner Mat B is thought of as an approximation (maybe a
> perfect one) to the operator A, 

Pmat (B) is just data used to construct the preconditioner.

> to be used to construct preconditioners. PCMAT uses B as an
> approximation to the *inverse* of A (loosely speaking). That's
> confusing, and worse it ruins a central feature of PETSc, which is the
> ability to change the preconditioner at runtime; the meaning of B
> changes depending on which PC implementation you use.

I agree that it cripples the ability to swap PC.

> Proposed change: Have PCMAT (like analogous object PCKSP) store its
> own Mat object and introduce API functions to interact with this
> internal Mat. To make sure that any user code which happens to use
> PCMAT doesn't see an unexplained change in behavior, do not set a
> default internal Mat, and produce an informative error if
> PCApply_Mat() is called without first calling the new function
> PCMatSetMat().

Interacting directy with the PC sucks for nested systems.  You could
have MatSetPC(mat,minv) and have PCMAT (or whatever) pull it out.
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