[petsc-users] A question on finite difference Jacobian

[Zou (Non-US), Ling]

On Tue, Oct 6, 2015 at 2:38 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Oct 6, 2015, at 3:29 PM, Zou (Non-US), Ling <ling.zou at inl.gov> wrote:
> >
> > Hi All,
> >
> > If the non-zero pattern of a finite difference Jacobian needs 20 colors
> to color it (20 comes from MatFDColoringView, the non-zero pattern is
> pre-determined from mesh connectivity), is it true that PETSc needs 40
> functions evaluation to get the full Jacobian matrix filled? This is
> because that a perturbation per color needs two function evaluation
> according to PETSc manual (ver 3.6, page 123, equations shown in the middle
> of the page).
> > But I only see 20 function evaluations. I probably have some
> misunderstanding somewhere. Any suggestions?
>
>    PETSc uses forward differencing to compute the derivatives, hence it
> needs a single function evaluation at the given point (which has almost
> always been previously computed in Newton's method)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Is it a potential problem if the user chooses to use a different (e.g.
simplified) residual function as the function for MatFDColoringSetFunction?



> and then one function evaluation for each color. This is why it reports 20
> function evaluations.
>
>   Barry
>
>
> >
> > Ling
>
>
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