[petsc-users] Poor multigrid convergence in parallel

[Jed Brown]

Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> writes:
> No, I'm L2-projecting (with mass-lumping) for the restriction.  So if I
> weren't lumping, I think this is the dual of the prolongation.  

A true L2 projection is a dense operation (involves the inverse mass
matrix).  But here, we're trying to map residuals, not states, so
_integration_ rather than "projection" is the desired operation.
Integration is the transpose of your prolongation.  We can do a precise
analysis, but I expect that you'll see slightly better convergence if
you modify this operator.

I would also recommend trying FMG soon.  You should be able to converge
in one F-cycle, and that will be faster than V-cycles with Krylov.
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