On 21 Jul 2014, at 11:50, Dave May <dave.mayhe...@gmail.com> wrote: > Hi Lawrence, > > Hmm this sounds odd. The convergence obtained with chebyshev should be > essentially identical in serial and parallel when using a jacobi > preconditioner
So I was maybe a bit unclear in my previous mail: If I run with: -pc_type mg -mg_levels_ksp_type richardson -mg_levels_pc_type jacobi -mg_levels_ksp_max_it 2 then I get identical convergence in serial and parallel if, however, I run with -pc_type mg -mg_levels_ksp_type chebyshev -mg_levels_pc_type sor -mg_levels_ksp_max_it 2 (the default according to -ksp_view) then I get very differing convergence in serial and parallel as described. > 1) How did you configure the coarse grid solver in the serial and parallel > test? Are they consistent? I initially just used the default (which is LU in serial and redundant with LU in parallel), when I rerun with: -pc_type mg -ksp_type fgmres -mg_coarse_ksp_type gmres -mg_coarse_pc_type jacobi -mg_coarse_ksp_max_it 100 Which should be the same in serial and parallel, I again see bad behaviour in parallel. > 2) Does using one level with PCMG and a chebyshev smoother give the same > answers in serial and parallel? If you precondition with jacobi, the > residuals in serial and parallel should be very similar. If this test didn't > pass, try a 1 level method again again without the preconditioner (i.e. just > apply chebyshev). If the residuals are really different between these runs, > there is likely something wrong/inconsistent with the definition of the > operator on each level, or the way the boundary conditions are imposed. For these tests, I use the following options: -pc_type mg -ksp_type fgmres -pc_mg_levels 2 -mg_coarse_ksp_type gmres -mg_coarse_pc_type jacobi -mg_coarse_ksp_max_it 100 I then tried the following options for the smoother: -mg_levels_ksp_type chebyshev -mg_levels_pc_type sor -mg_levels_ksp_max_it 2 Works in serial, doesn't converge well in parallel. -mg_levels_ksp_type chebyshev -mg_levels_pc_type jacobi -mg_levels_ksp_max_it 2 Converges veery slowly in both serial and parallel (but has the same convergence in both cases). -mg_levels_ksp_type chebyshev -mg_levels_pc_type none -mg_levels_ksp_max_it 2 Converges well in both serial and parallel (identical convergence behaviour). > 3) Is the code valgrind clean? It's python-based, so it's a little difficult to say. It appears to be so. Cheers, Lawrence
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