To follow up, On 21 Jul 2014, at 13:11, Lawrence Mitchell <lawrence.mitch...@imperial.ac.uk> wrote:
> > On 21 Jul 2014, at 12:52, Dave May <dave.mayhe...@gmail.com> wrote: > >> >> -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 >> >> >> Good. That's the correct result. >> >> 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. >> >> >> It's normal that the behaviour is different. The PETSc SOR implementation is >> not parallel. It only performs SOR on your local subdomain. I think I've convinced myself that I was just getting unlucky with the chebyshev+sor smoothing combination when I looked, modifying the sor weighting, or choosing something other than 2 processes for the particular problem I was seeing bad 2-process convergence on, gives good convergence again. I'll keep an eye on it in case it turns out I have been doing something stupid. Thanks for the various pointers on debugging though. Cheers, Lawrence
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