On May 9, 2013, at 4:00 PM, Dmitry Karpeev <[email protected]> wrote:
> GAMG is primarily an algebraic multigrid preconditioner. > The geometric part is somewhat weaker and isn't the primary purpose of GAMG, > so I would recommend using -pc_gamg_type agg for plain aggregation. > OK that's clear know, I was asking about the low level mesh implementation because I thought the geometric part will need to know something about the coordinates and what not. > For good performance with elasticity it is important to use smoothed > aggregation, > for example, -pc_gamg_agg_nsmooths 1 > but for that to be effective, it is critical to supply a "nearnullspace" -- > basically, > the nullspace of the Neumann operator. It is the rigid body modes in the > case of elasticity. > If you are able to specify the block size of the matrix, GAMG will try to > guess the correct > nearnullspace for you. In particular bs=3 will cause GAMG to construct the > rigid body modes > from the coordinates set with PCSetCoordinates. > > Now, libMesh doesn't have an API for setting the matrix block size (as far as > I know). > You can hack a bit: extract the PETSc objects and do it yourself, but a more > natural way is > to compute your own rigid body modes in a callback attached to > NonlinearSolver::nearnullspace or NonlinearSolver::nearnullspace_object. A > nearnullspace is simply an array of NumericVector that > contains a basis for the nearnullspace. libMesh will orthonormalize it for > you (PETSc requires > that). > Crystal clear. I'll get to it. Best, Ata > Hope this helps. > Dmitry. > > > On Thu, May 9, 2013 at 11:08 AM, Cody Permann <[email protected]> wrote: > On Thu, May 9, 2013 at 9:18 AM, Ataollah Mesgarnejad <[email protected] > > wrote: > > > > > Cody, > > > > On May 9, 2013, at 9:29 AM, Cody Permann <[email protected]> wrote: > > > > > > > > > > On Thu, May 9, 2013 at 8:07 AM, Ataollah Mesgarnejad < > > [email protected]> wrote: > > > >> Dear all, > >> > >> I was wondering if anyone has any experience using gamg with libMesh (I'm > >> particularly interested in elasticity)? > >> > >> > > It certainly works, however in my very limited experiecne, it's slightly > > more tricky to use than Hypre as we've found that it requires a few more > > options. Gamg is very flexible and part of PETSc so it ties in nicely. > > > > > > > > That's wonderful. Do I need to add anything in the code itself like block > > size or ..? Or do you mean the command line options for gamg? > > > > Just the command line options. If you have specific questions, the petsc > user list is a great place to ask. > > Cody > > > > > My main question would be if the geometric part of gamg would work with > >> the underlying mesh structure in libMesh? > >> > > > > I'm not sure if I'm missing something with this question. I don't see why > > this preconditioner would care about the mesh structure, it's abstracted > > well above that level. From libMesh's perspective, it's just another > > preconditioner. > > > > > > OK. I'm not very familiar with low level structure in libMesh so I wasn't > > sure. > > > > Thanks, > > Ata > > > > > > Cody > > > > > >> > >> I would appreciate any comments/recommendations, > >> Ata > >> > >> ------------------------------------------------------------------------------ > >> Learn Graph Databases - Download FREE O'Reilly Book > >> "Graph Databases" is the definitive new guide to graph databases and > >> their applications. This 200-page book is written by three acclaimed > >> leaders in the field. The early access version is available now. > >> Download your free book today! http://p.sf.net/sfu/neotech_d2d_may > >> _______________________________________________ > >> Libmesh-users mailing list > >> [email protected] > >> https://lists.sourceforge.net/lists/listinfo/libmesh-users > >> > > > > > > > ------------------------------------------------------------------------------ > Learn Graph Databases - Download FREE O'Reilly Book > "Graph Databases" is the definitive new guide to graph databases and > their applications. This 200-page book is written by three acclaimed > leaders in the field. The early access version is available now. > Download your free book today! http://p.sf.net/sfu/neotech_d2d_may > _______________________________________________ > Libmesh-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/libmesh-users > > > > ------------------------------------------------------------------------------ Learn Graph Databases - Download FREE O'Reilly Book "Graph Databases" is the definitive new guide to graph databases and their applications. This 200-page book is written by three acclaimed leaders in the field. The early access version is available now. Download your free book today! http://p.sf.net/sfu/neotech_d2d_may _______________________________________________ Libmesh-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libmesh-users
