[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Dmitry Karpeev]

Good point.  It is still strange, however, that SNES insists on using the
same Mat obtained from DMGetMatrix for J and P.
Dmitry.
On Thu, Feb 9, 2012 at 11:46 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Thu, Feb 9, 2012 at 23:41, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>
>> Currently DMGetMatrix(DM dm, MatType mattype , Mat *J)  is supposed to
>> produce the system Jacobian, according to the docs.
>> SNES will use it, but, since DMGetMatrix returns a single matrix, it
>> can't use a different preconditioning matrix.
>> Why not change this to DMGetMatrices(DM dm, MatType jtype, Mat *J,
>> MatType  ptype, Mat *P)?
>> I'm not exactly sure what the best way of handling MatType should be
>> (same MatType for J and P, separate types, how dm->mattype
>> affects the result), but I think it can be sorted out, whereas the
>> absence of P is a clear limitation.
>>
>
> Umm, this doesn't put in values, it just creates a matrix of the right
> size. You can make two if you want two different matrices.
>
> If you are talking about DMDAFormJacobianLocal(), that does actually
> restrict what you can do, and you have to hack it a bit to use two
> different matrices (except by MFFD because the library specifically
> supports that).
>
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