[petsc-dev] Jed!!!

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>   I am totally confused and likely users are to. Please point me to an
>   example that actually uses Matnest the way it is “supposed to be
>   used”. 

src/snes/examples/tutorials/ex28.c

Note that this does not contain the word "nest".  That is as intended.

>   I cannot even find a MatSetValues_Nest() so I do not see how one can
>   even interchange code between AIJ and Nest. 

You're supposed to use MatGetLocalSubMatrix().  Translating from global
indices to local indices is a disaster that we want to avoid.  So we go
the other way.  Speak the language of "split local" spaces during
assembly and the data structure itself can live wherever is most
efficient for the solver.

>   The users manual has the word nest several times but no indication
>   of how to use the damn thing.

From the manual:

  The key to format-independent assembly is the function

  MatGetLocalSubMatrix(Mat A,IS isrow,IS iscol,Mat *submat);
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