VecGhost and PCFieldSplit

[Jed Brown]

Gee, having vectors with a little extra space at the end is hard!

Notation: J is the Jacobian, P is the associated preconditioning matrix

PCFieldSplit gets the full row blocks (Afield) or the off-diagonal
blocks (B,C) from J.  I implement these matrix-free and use VecGhost
internally.  It also queries P for diagonal blocks (A,D).  I provide
these as assembled AIJ and BAIJ matrices.  It then obtains work vectors
by getting vecs from A and D which returns ordinary MPI vectors (no
ghosts).  It then tries to multiply B and C with these vectors which
fails because I need the ghosts.  In my particular case, this could be
fixed by getting the work vecs from B and C, but that just pushes the
problem elsewhere.

So it looks like I either have to abandon VecGhost (which I'd rather not
do because it would add two copies to every matrix-free multiply, and
these multiplies normally only touch a little more memory than a vector
an thus can be pretty cheap) or I need to be able to turn an unghosted
vec into a ghosted one (in-place since I have a Vec, not a Vec*).  I can
think of a couple ways to do this, but they would be a maintenance
nightmare.  There might be a way to reorganize
VecCreateGhostBlockWithArray to make this possible, but it still looks
messy to me.  Any ideas?


Jed

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