[petsc-users] MatGetSubMatrix for a Nested matrix

[Jed Brown]

Khai Hong Pham <khaipham at utexas.edu> writes:

> Thank you for your information, Matt. I'm using both nested matrix and
> nested vector. Is the nested vector good to use ?

VecNest is far worse than MatNest.  Do not use it under any
circumstances.  Less than 1% of people that think they have run out of
options and absolutely must use VecNest have any business using it.

Always make your code work without MatNest first.  Check that the
answers make sense, then profile to see where memory and time is spent.
If you are preconditioning with fieldsplit and excessive memory and time
is spent in MatGetSubMatrix(), use -prefix_mat_type nest as an
optimization.
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