On Mar 20, 2013, at 6:14 PM, John Mousel <john.mousel at gmail.com> wrote:
> Can you comment on a GASM type approach to find a solution for the null > space? I notice that the null vectors that successfully make the true > residual drop are only complicated in a very thin band around the interface. > This band is easy to identify using a level set. Other than that, the null > space vector has a low frequency variation. My thought was to break the > matrix into two sub-matrices, and somehow apply GAMG as a preconditioner on > the far matrix, and ILU on the interface-adjacent matrix. Is this dumb or a > complete misunderstanding of GASM? > The null space in GAMG is not a projection, it does not have to be exact. Do you try using the 6 RBM or giving GAMG coordinates? Also, you might try not smoothing the (-pc_gamg_nsmooths 0). Unsymetric matrices can work better this way. > > On Wed, Mar 20, 2013 at 5:08 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote: > > On Wed, Mar 20, 2013 at 5:04 PM, John Mousel <john.mousel at gmail.com> wrote: > I've wanted to scrap this approach for a long time, but moving away from > these GFM-type treatments is not a choice that I've been allowed to follow > through on for various reasons which are out of my control. > > Unless there are some clever tricks to characterize the null space or to keep > preconditioners compatible with the null space, the folks making the > decisions might have to reconsider. It doesn't matter how sexy a method looks > if it requires a solve and that solve cannot be done efficiently. > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130320/f0718809/attachment.html>
