Hi,, Thanks for more details!! Recursive two level multigrid? (by using PCMG with my desired stuff in it, in PCSHELL) is okay. I will do it recursively. No Problem with recursion and nested iterations. Coming to two level, How can I get exactly Prec = C + ?M^-1 (I - A*C) in a two level multigrid ? What should be specific choice of pre_smoother_ksp_context and post_smoother_ksp_context to get the "Prec" ? what operators should be chosen in these two ksp contexts ?
Awiaitng, with regards, Abdul >________________________________ > From: Jed Brown <jedbrown at mcs.anl.gov> >To: PETSc users list <petsc-users at mcs.anl.gov> >Cc: Abdul Hanan Sheikh <hanangul12 at yahoo.co.uk> >Sent: Thursday, 5 April 2012, 17:56 >Subject: Re: [petsc-users] Implementation of Multilevel preconditioner > > >On Thu, Apr 5, 2012 at 08:38, Barry Smith <bsmith at mcs.anl.gov> wrote: > >? ?C + ?M^-1 (I - A*C) ? is exactly a multiplicative Schwarz method with two >preconditioners. Or in another way to put it it is exactly a two level >multigrid so you can use PCMG in a straightforward way. > >It looks to me like he wants to do nested iteration with recursive coarse >levels rather than just iteration in a smoother. > > >I need the accuracy at first and somehow at second coarse level.? >After these two levels, the number of iterations at each level till coarsest >is constantly one. ? > > >Okay, but one iteration of a preconditioned Krylov method is still two >preconditioner applications, so the method would still be exponential in the >number of levels. If Richardson iteration is okay, you can use >-pc_mg_cycle_type w to do two iterations instead of 1. The code would actually >work fine with increasing this further, but it looks like you would currently >have to cheat a bit to get many recursive cycles. > > >PCMGSetCycleType(pc,(PCMGCycleType)5); > > >There is currently no interface for setting a different number of cycles on >each level, but you could include mgimpl.h and change the levels->cycles >manually. > > >Note that W cycles are already often considered to be too much work on coarse >levels to justify in a parallel setting, so it's somewhat unlikely that >running more coarse level iterations will really be better than a more >standard multigrid with stronger smoothers. > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120405/a5b62f3e/attachment.htm>
