> On Sep 23, 2017, at 12:48 PM, Mark W. Lohry <[email protected]> wrote:
>
> I'm currently using JFNK in an application where I don't have a hand-coded
> jacobian, and it's working well enough but as expected the scaling isn't
> great.
>
> What is the general process for using PC with MatMFFDComputeJacobian? Does it
> make sense to occasionally have petsc re-compute the jacobian via finite
> differences, and then recompute the preconditioner? Any that just need the
> sparsity structure?
Mark
Yes, this is a common approach. SNESSetLagJacobian -snes_lag_jacobian
The normal approach in SNES to use matrix-free for the operator and use
finite differences to compute an approximate Jacobian used to construct
preconditioners is to to create a sparse matrix with the sparsity of the
approximate Jacobian (yes you need a way to figure out the sparsity, if you use
DMDA it will figure out the sparsity for you). Then you use
SNESSetJacobian(snes,J,J, SNESComputeJacobianDefaultColor, NULL);
and use the options database option -snes_mf_operator
> Are there any PCs that don't work in the matrix-free context?
If you do the above you can use almost all the PC since you are providing an
explicit matrix from which to build the preconditioner
> Are there any example codes I overlooked?
>
> Last but not least... can the Boomer-AMG preconditioner work with JFNK? To
> really show my ignorance of AMG, can it actually be written as a matrix
> P^-1(Ax-b)=0, , or is it just a linear operator?
Again, if you provide an approximate Jacobian like above you can use it with
BoomerAMG, if you provide NO explicit matrix you cannot use BoomerAMG or almost
any other preconditioner.
Barry
>
> Thanks,
> Mark
