[petsc-users] MATNEST with shell matrices

[Amneet Bhalla]

Jed, so to use VecGetSubVector() with a regular Vec; it would need to set
Vec->map->bs = no_of_components
that I want to solve for? I understand that a wrapper code needs to be
written for gelling SAMRAI data field and PETSc Vec; I am referring to such
vector wrapper here.

On Thu, Feb 21, 2013 at 5:33 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

>
> On Thu, Feb 21, 2013 at 5:12 PM, Boyce Griffith <griffith at cims.nyu.edu>wrote:
>
>> Sorry I couldn't jump in earlier --- I had a proposal going out today and
>> am just now catching up on other email.
>>
>> Amneet is trying to solve a system the couples together some field data
>> that is described using SAMRAI data structures along with particles that
>> are stored in a standard PETSc Vec.  The hook between the SAMRAI data and
>> PETSc is via a version of Bobby Philip's SAMRAI-to-PETSc interface that
>> I've bastardized for my own purposes.  There is no support for
>> {Mat,Vec}SetValues here --- the wrapper just provides standard vector space
>> operations required by Krylov and Newton-Krylov solvers.
>>
>
> Boyce, thanks for explaining. We *always* need to know this sort of
> background in order to make useful suggestions.
>
>
>>
>> I used to have some code to make collections of Mat's and Vec's to make
>> it feasible to solve systems involving both native and non-native PETSc
>> data structures.  (It was similar to some code for making Vec's of Vec's
>> that someone --- Dave May? --- used to maintain.)  I ripped this stuff out
>> of my code a while ago, and I suggested that Amneet investigate
>> {Vec,Mat}Nest before we tried to resurrect that old code.
>>
>
> That was called PetscExt. Dave and I wrote MatNest out of desire to do the
> sort of things that PetscExt did in a way that was more flexible and would
> interoperate better with the rest of PETSc. It, and PCFieldSplit should be
> a complete replacement for anything you might have used PetscExt for in the
> past.
>
> If you already have MatShells around for all the blocks, you can easily
> wire them together with MatNest.
>
> Note that MatNest does *not* depend on VecNest, and in general, I
> recommend not using VecNest. If you call VecGetSubVector() with a
> contiguous index set, you'll get a subvec back without a copy. This means
> that your residual evaluation only needs to provide a map from your
> non-contiguous local representation to a contiguous global representation.
> This should be no overhead and will interoperate better.
>
> But if you really want to use those other vector space wrappers, you can
> use VecNest.
>



-- 
Amneet
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