On Mon, Oct 12, 2015 at 9:11 PM, K. N. Ramachandran <[email protected]> wrote:
> Hello Matt, > > Actually I felt the boundary conditions were having a role to play and set > all the boundary conditions to Dirichlet. In this case, convergence was > almost immediate with the Hypre preconditioner, taking 17 seconds with 3 > iterations. The MG method took about the same time though. > > So I reverted to the Dirichlet, Neumann mix of BCs and Hypre starts to > diverge. Please find attached the output for the Hypre run using Dirichlet > and Neumann for a 21^3 grid (rows), with a max of 7 nonzeros per row. > Details of the options used before running are in the file. The solver used > in all cases is bcgs. > > Also attached is the MG output for 101^3 grid > 1) Dirichlet and Neumann > 2) Dirichlet only > > where it seems to take about the same time. > Notice that you have no levels of MG here. You need to use -pc_mg_levels <n> Matt > I was thinking if the Null space has something to do with this? From the > PETSc docs, I understand that the Null space should be removed/attached > when solving singular systems. My domain will have at least two Dirichlet > conditions, so it is not singular. But since the Neumann conditions seem to > affect the convergence with Hypre, perhaps they have a role to play in > terms of the null space? > > > > On Mon, Oct 12, 2015 at 3:52 PM, K. N. Ramachandran <[email protected]> > wrote: > >> Hello Matt, >> >> Thanks for your reply. I am actually retrying against the latest PETSc >> and will revert once I get the information. >> >> On Mon, Oct 12, 2015 at 9:39 AM, Matthew Knepley <[email protected]> >> wrote: >> >>> On Sat, Oct 10, 2015 at 7:56 PM, K. N. Ramachandran <[email protected]> >>> wrote: >>> >>>> Sorry, some more questions. >>>> >>>> 3) Also, for Dirichlet bc, I specify the value through Identity rows, >>>> i.e. A_ii = 1 and the rhs value would correspond to the Dirichlet >>>> condition. I am specifying it this way for my convenience. I am aware that >>>> MatZerosRowColumns might help here, but would keeping it this way be >>>> detrimental? >>>> >>> >>> That is fine. However you would want to scale these entries to be >>> approximately the same size as the other diagonal entries. >>> >>> >>>> 4) Can I expect symmetric matrices to perform better, i.e. if I >>>> eliminate Dirichlet rows? But I would still be left with Neumann boundary >>>> conditions, where I use the second order formulation. If I used the first >>>> order formulation and made it symmetric, would that be an advantage? I >>>> tried the latter, but I didn't see the condition number change much. >>>> >>> >>> Will not matter for MG. >>> >>> Matt >>> >>> >>>> >>>> >> >> Regards, >> K.N.Ramachandran >> Ph: 814-441-4279 >> > > > > Thanking You, > K.N.Ramachandran > Ph: 814-441-4279 > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
