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/20120405/88af584a/attachment.htm>
