[petsc-dev] getting data out of a PetscFE simulation

[Geoffrey Irving]

On Wed, Jan 29, 2014 at 12:00 PM, Jed Brown <jed at jedbrown.org> wrote:
> Local vectors are supposed to be just that: local. VecViewing a local vector and expecting it to be parallel is perverse.  So we need a real interface.
>
> Placing 1.0 on the diagonal (and don't assemble into those rows and columns) is the common way to deal with Dirichlet boundary nodes. See ex48 for one example. I have written about this in a few places; I can find the more complete description when I have a keyboard.

I'll look forward to the improved interface.  For better or worse, I'd
like to be able to view simulations in the near future, so it looks
like I have to go with Matt's perverse but working version now.

Geoffrey

> On January 29, 2014 12:53:26 PM MST, Geoffrey Irving <irving at naml.us> wrote:
>>On Wed, Jan 29, 2014 at 11:36 AM, Jed Brown <jed at jedbrown.org> wrote:
>>> Matt's sample code doesn't set it either. We need to fix (and the
>>*only* acceptable fix if that VecView always does the right thing,
>>because we have to be able to call it in analysis settings that know
>>nothing about your discretization).
>>
>>Matt's sample code doesn't set it either, but for Matt's sample code I
>>know where to insert the one line call to DMPlexProjectFunctionLocal.
>>For your version I never have explicit access to the local vector, so
>>I can't insert the fix.
>>
>>> The problem is that some vectors reside in a homogeneous space (e.g.
>>increments and eigenvectors) while others reside in the inhomogeneous
>>space (solutions). We can add a flag or BC attribute on the vector to
>>this effect, but this (and slip conditions) was the issue that led me
>>to conclude that removing boundary nodes was mostly a false economy.
>>
>>To leave the boundary conditions in, we would need efficient support
>>for a very large, very sparse MatNullSpace.  This is doable via
>>shells, but is it easy to do in a way that doesn't interfere with the
>>user's other null spaces?
>>
>>Geoffrey
>