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
