[petsc-dev] PCGAMG: set default type (AGG); PCSetUp_GAMG crashes if not set

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:
> So the rationale for this is when we change backend types?

Yes, when we changed implementations, we shouldn't be contaminated by
whatever came before, but if the user does

PCSetType(pc,PCASM);
PetscObjectComposeFunction(pc,"TheirFunction",TheirFunction);
PCSetType(pc,PCGAMG);

Their function should not be removed.  Thus the thought of clearing
based on a prefix.

> Is it worth anything more than a convention? If so, what about just having
> a count in PetscObject that gets checked in the teardown process.

1. How do we check the convention?  It's very error-prone at present.

2. See above for why a simple count is not good enough.
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