[petsc-dev] [petsc-users] unreliable AMG in PETSc

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>    How can we insure that that the smoother is SPD? Which are known to be. In his case it was Richardson + SSOR on a bunch of processes.

The Richardson scaling cannot be too ambitious and the SSOR needs to be
an SPD operator.  I think this is true any time the forward and back
sweep is stable, which is guaranteed for an M-matrix, for example.

I'm not aware of an inexpensive way to guarantee stability/SPD smoother
for a general matrix, and even if you could choose parameters to make it
so, you might end up with such aggressive under-relaxation that the
solver configuration is not useful.

Focus on debuggability or automation?
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