[petsc-users] MATNEST with shell matrices

[Jed Brown]

On Thu, Feb 21, 2013 at 7:28 PM, Boyce Griffith <griffith at cims.nyu.edu>wrote:

>
> I would very happily use an "undefined" vector size.  (I will also happily
> change the bogus local sizes from 0 to 1, and the global sizes accordingly.
> :-)
>
> The PETSc solvers that we use in the code should be oblivious to the size
> of the vector.  I've actually never been all that happy with size "0", but
> it was in some inherited code, and at least so far, it has never caused any
> problems over the 10+ years it has been in use.  (If ever did start causing
> problems, I certainly would not come complaining to petsc-users or
> petsc-dev.  It isn't like I do not realize that "0" is a potentially
> problematic value here.)
>
> I actually have always hoped that setting size to "0" would help smoke out
> any places where something actually did use the vector size.
>

Okay, let's move the design discussion to petsc-dev.

I want to understand the extent to your adaptivity and how your Vecs
"behave". In which places do you (mathematically) change the dimension of a
vector?

If I start with Vecs X and Y and I write

VecCopy(X,Y);
YourSolver(X); /* solve F(X) = 0 with adaptivity */

can I then compare the result like this?

VecAXPY(Y,-1.,X);

Do vector-space properties of Y (e.g., norm) change when I call
YourSolver(X). (Some codes keep a global view of all vectors, and when they
adapt, they regrid all vectors. But regridding doesn't preserve things like
norms, so you end up with tangled semantics.)
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