On Thu, Sep 19, 2013 at 2:53 PM, Barry Smith <bsm...@mcs.anl.gov> wrote:
> > It is essentially 3 Laplacians so I think the default null space of 3 > constant vectors is fine. The problem is without the block information > presumably GAMG is using only a single constant vector over all variables? > So maybe we need to construct a 3 vector null space which just marks in > the reduced vector from which of the 3 components each entry came from. > SNESVIGetActiveSetIS() currently doesn't try to detect or set the inactive set IS block structure, so MatGetSubMatrix() can't take advantage of that. This (blocking) information is geometric in nature -- since it is mesh nodes that really come "into contact" -- so unless we involve a DM somehow, I don't know if it is possible to retain the right blocking structure in general. Or add a user callback hook to compute the inactive set? > Barry > > On Sep 19, 2013, at 3:48 PM, Jed Brown <jedbr...@mcs.anl.gov> wrote: > > > Barry Smith <bsm...@mcs.anl.gov> writes: > >> That is what it is doing and apparently it doesn't result in a good > >> preconditioner; I don't know why off hand. One thing is it no > >> longer knows about the block structure. > > > > How is the near null space being specified? > >