[petsc-users] MATNEST with shell matrices

[Jed Brown]

On Thu, Feb 21, 2013 at 6:12 PM, Boyce Griffith <griffith at cims.nyu.edu>wrote:

> But there is a local and global size. (You had to pick something to
>> create the Vec.) Therefore, the global space is _defined_ as the
>> concatenation of all those contiguous local spaces. You need not store
>> it that way, but the abstract space exists. If you use interfaces that
>> ask for ISs, they will be with respect to that abstract space.
>>
>
> Ah, I think I wasn't clear about what I mean here by "wrapper".
>
> Basically, we have a subclass of Vec that knows how to evaluate vector
> space operations using the "wrapped" SAMRAI data.  For instance, there is
> no VecGetArray().



I understand that completely, but you cannot create a valid Vec without
setting a size. Internally the PetscLayout has a size of -1 on creation and
you will get errors if you try to use it without setting a size. Setting a
size of 0 is patently dishonest and PETSc could justifiably short-circuit
and not call into your implementation. (I don't think we do this, but it
would be correct at the vector space level. PETSc Vec is a
finite-dimensional thing, therefore it has a well-defined dimension.) I'm
saying that you should be honest about the size even if it's not explicitly
used in the functionality you happen to be using now. If you define a size,
then you can also (optionally) use IS to refer to pieces, which will make
interoperability with other codes, or eventual extensions (such as AMG
preconditioning) less intrusive.
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