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? Are there any PCs that don't work in the matrix-free context? 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? Thanks, Mark
