[petsc-dev] coarse grid in gamg (for the OPTE paper)

[Dmitry Karpeyev]

On Thu, Sep 19, 2013 at 2:53 PM, Barry Smith <bsmith at 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 <jedbrown at mcs.anl.gov> wrote:
>
> > Barry Smith <bsmith at 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?
>
>
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