[petsc-users] (no subject)

[Jed Brown]

Mark Adams <mfadams at lbl.gov> writes:

> FM matrices are slow and meant for debugging mostly (I thought,
> although the docs just give this warning if coloring is not available).
>
> I would check the timings from -log_view and verify that the time is spent
> in MatFDColoringApply. Running with -info should print the number of colors
> (C). The cost of an FD matrix is about C x cost of an exact Jacobian.
> Roughly. You could check that.

This depends a lot on how efficient the residual and Jacobian evaluation
is.  We've seen examples where a colored Jacobian is less expensive than
analytic, even when the analytic is not "poorly written".