[petsc-users] MatMatMult involving MPIAIJ and MATNEST

[Jed Brown]

Bikash Kanungo <bikash at umich.edu> writes:

> The physics of the problem requires evaluation of M^{-1} from four
> sub-matrices. That's why I'm constrained to use it as MATNEST. The other
> option seems to be MatGetLocalSubMatrix but it seemed easier for me to go
> with MATNEST (particularly to avoid the local index sets).

If you need a matrix-matrix multiply, then artificial intermediate
scales will tend to reduce performance rather than improving it.

We can implement MatMatMult_Nest_Nest, but based on what you've said, I
doubt it's a good fit for your problem.  MatMatMult_Nest_MPIAIJ would
likely perform worse than MatMatMult_MPIAIJ_MPIAIJ.
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