[petsc-users] snes + matrix free + coloring

[Jed Brown]

"Zou (Non-US), Ling" <ling.zou at inl.gov> writes:

> Hi Jed,
>
> I was searching a topic on preconditioning in the mail list and found this
> email.
>
>
> On Wed, Oct 10, 2012 at 2:55 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> Then you aren't using a matrix-free method at all.
>
>
> In case of using SNESDefaultComputeJacobianColor to compute Jacobian matrix
> and mf_operator command, is this exactly what you are describing ('aren't
> using a matrix-free method at all')?
>
> The SNESDefaultComputeJacobianColor I found in lines 149-155 from
> http://www.mcs.anl.gov/petsc/petsc-2.3.3/src/snes/examples/tutorials/ex5.c.html

1. Please upgrade PETSc.  I don't think we have any links to these
antique man pages, so I assume you specifically changed the version
number back rather than landing there accidentally.

2. It is NOT a matrix-free method because coloring is used to assemble
the matrix.  It usually doesn't make sense to assemble a matrix using
coloring, but then not use it to apply the matrix (-snes_mf_operator
only uses the assembled matrix for preconditioning).
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