Dear Jed and Mark. Thank you for your quick answers. 1. I am stuck to petsc-3.2 since I am using PETSc in a production software. For the next release, I'll give GAMG a try (I have been a satisfied user of Prometheus). 2. It seems to me that ML does not support block matrices, doesn't it? 3. I would like to give ML the information about the vector nature of my problem in order that all 3 dofs of a given node be treated in the same manner (in the coarsening phase for instance).
Nicolas 2012/4/2 Mark F. Adams <mark.adams at columbia.edu> > FYI, you can try GAMG also. It uses the same algorithm as ML: > > Here are parameters: > > -pc_type gamg > -pc_gamg_type agg > -pc_gamg_agg_nsmooths 1 ! this is good for elliptic problems > -pc_gamg_verbose 2 ! this will help me to give a quick > sanity check > -pc_gamg_threshold .05 ! for 3D problems this parameter can be > useful for optimization > -pc_gamg_coarse_eq_limit 50 ! a detail but you can keep in your input deck > > The vector Lapacian has 6 null space vectors (3 translational, 3 > rotational). GAMG will construct these with the a setCoordinates method > (see the ksp tutorial example ex56.c). If you prefer to give me the null > space vectors explicitly there is the MatSetNearNullSpace() method that Jed > mentioned. GAMG has not implemented this but I should at some point so if > you want to use it I can implement it. > > If you do not give it coordinates it will construct the 3 translational > null space vectors (don't need anything to compute these) and it can be OK. > At least a good place to start. > > Mark > > On Apr 2, 2012, at 10:44 AM, Jed Brown wrote: > > On Mon, Apr 2, 2012 at 06:59, Karin&NiKo <niko.karin at gmail.com> wrote: > >> I would like to use ML as a preconditioner for a linear elasticity >> problem. >> I wonder if there is a way to indicate ML that it is dealing with a >> vector problem. >> I know that when using Prometheus, one must use a block matrix, and I >> would like to know if there is a trick >> in order to use ML in an optimal manner for vector problems. >> > > The block size is used if you don't provide any other information. The > preferred approach with petsc-dev is to use MatSetNearNullSpace(). (I'll > make sure this is currently working with ML and reply to this message.) > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120402/67a9a7ab/attachment.htm>
