[petsc-users] MatNest with Shell blocks for multipysics

[Matteo Semplice]

Il 01/07/21 17:52, Jed Brown ha scritto:
> I think ex28 is better organization of code. You can DMCreateMatrix() and then set types/preallocation for off-diagonal blocks of the MatNest. I think the comment is unclear and not quite what was intended and originally worked (which was to assemble the off-diagonal blocks despite bad preallocation).
>
> https://gitlab.com/petsc/petsc/-/commit/6bdeb4dbc27a59cf9af4930e08bd1f9937e47c2d
>
> https://www.mcs.anl.gov/petsc/petsc-current/src/snes/tutorials/ex28.c.html#line410

Thanks! Yesterday I was unable to make it work, but I'll have another go 
with ex28 then...

> Note that if you're using DMDA and have collocated fields, you can skip all this complexity. And if you have a scattered discretization, consider DMStag. ex28 is showing how to solve a coupled problem where there is no suitable structure to convey the relation between discretizations.

The current discretization is in fact colocated with the variables on 
the same grid, and we might stick to that for a while.

However, one key point in the design is that the jacobian will be 
[A00,A01;A10,A11] and we already have a taylor-made shell preconditioner 
for A00 and A00 implemented as a shell matrix; the preconditioner for 
the full Jacobian will be to neglect the A10 block and do a 
block-triangular solve inverting A00 approximately with the shell 
preconditioner.

I do not understand how creating a DMDA with n0+n1 dofs will let me 
easily reuse my shell preconditioner code on the top-left block.

Matteo