[petsc-users] GAMG parallel convergence sensitivity

[Mark Adams]

>
>
>
> Any thoughts here? Is there anything obviously wrong with my setup?
>

Fast and robust solvers for NS require specialized methods that are not
provided in PETSc and the methods tend to require tighter integration with
the meshing and discretization than the algebraic interface supports.

I see you are using 20 smoothing steps. That is very high. Generally you
want to use the v-cycle more (ie, lower number of smoothing steps and more
iterations).

And, full MG is a bit tricky. I would not use it, but if it helps, fine.


> Any way to reduce the dependence of the convergence iterations on the
> parallelism?
>

This comes from the bjacobi smoother. Use jacobi and you will not have a
parallelism problem and you have bjacobi in the limit of parallelism.


> -- obviously I expect the iteration count to be higher in parallel, but I
> didn't expect such catastrophic failure.
>
>
You are beyond what AMG is designed for. If you press this problem it will
break any solver and will break generic AMG relatively early.

This makes it hard to give much advice. You really just need to test things
and use what works best. There are special purpose methods that you can
implement in PETSc but that is a topic for a significant project.
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