Justin Chang <jychan...@gmail.com> writes: > So when would I use one over the other? > > - If I wanted to solve a problem using a direct solver or an iterative > solver without a preconditioner, I would use A = J? > > - The documentation for SNESSetJacobian() says that AMat and PMat are > usually the same, but if I used something like GAMG would I need to create > two different objects/Mats?
It is a semantic distinction that has nothing to do with the preconditioning algorithm. If you define the operator that you want to solve with using a matrix-free implementation (could be MFFD, could be some fast evaluation that doesn't store matrix entries), but want to use an assembled operator for preconditioning, then you pass a different matrix. If you use a different discretization to define the operator versus the preconditioner, then you would pass different matrices. This happens, for example, when preconditioning using a low-order discretization and/or dropping some coupling terms that are deemed unimportant for preconditioning.
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