[petsc-dev] Questions about Petsc

[Jed Brown]

Please send these questions to petsc-dev (Cc'd now) or petsc-users so
that others can comment.

Lulu Liu <lulu.liu at kaust.edu.sa> writes:

> Hi,
>
> I want to implement ASPIN-like ideas, but based on field splitting. You
> could find the details in PDF file.

You are proposing left-preconditioned nonlinear additive fieldsplit.
This is a sensible algorithm that we have discussed before.  The only
part that needs to be implemented is the fieldsplit solve: separation of
G and H, then solving each separately.

Unfortunately, I think it's kinda tricky to come up with a generally
useful interface.  In particular, many applications cannot partition the
variables like you have done, meaning that the preconditioner is
becomes: change variables, solve in other bases, then change back.  At
this point, I would recommend implementing the solve with \hat F using
SNESShell.

> My questions:
>
> I have a nonlinear system including two equations. In my implementation, I
> need to solve each equation separately, also I need to solve the coupled
> system. I feel confused about ordering:
>
> I know the unknowns are ordered as [p1,s1,p2,s2,p3,s3,......] in PETSc,
> however, it seems that I need the variables like [p1,p2,p3,... s1,s2,s3,...]

No, just use a strided IS to describe each set of variables.  You can
VecScatter from the packed/monolithic global vector to the split space.

> Is there any example to do the similar things in PETSc?
> Do you have any ideas to implement the algorithm easier?
> How to quickly extract [p1,p2,...p_{N}] or [s1,s2,....] from
> [p1,p2,....s1,s2,s3,...] ?
> How to quickly get the composed [p1,p2,...s1,s2,....] if I have [p1,p2,...]
> and [s1,s2,..]?
>

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