[petsc-users] Poor multigrid convergence in parallel

[Lawrence Mitchell]

To follow up,

On 21 Jul 2014, at 13:11, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> wrote:

> 
> On 21 Jul 2014, at 12:52, Dave May <dave.mayhem23 at 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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