[petsc-dev] Lagging the numerical factorization for one of the fields of fieldsplit preconditioner

[Jed Brown]

On Wed, Mar 16, 2011 at 00:01, Barry Smith <bsmith at mcs.anl.gov> wrote:

> When a submatrix set is obtained from an AIJ it could mark the ranges the
> submatrices, row and column, then when MatSetValues is called in the future
> on the big matrix it could check what box the values are being put in and
> any box that has none put in is unchanged. For this to be efficient the
> submatrices have to be truly submatrices not just logically submatrix.
>

[A B; C D]

Two cases:

1. If you only modify A, then the columns of B will still be marked
unmodified so you know that B has not been changed so the cheap flag works.

2. Modify B and C, now the rows and columns of A and D will both be marked
as modified. The cheap flag doesn't work.


Also, if any of these blocks is a finite element assembly, the user will
typically zero the matrix before assembly. (They can avoid it by doing a
traversal with MatSetValues(...,zero,INSERT_VALUES), then an assembly, then
run over it again with ADD_VALUES, but that's awfully cumbersome.)


>  But we should muck around on this a bit more before we figure out how we
> want to handle the propagation of this kind of information efficiently and
> automatically.
>

Agree
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