[petsc-users] GAMG PC dependence on right-hand side

[John Mousel]

Mark,

I compared the eigen estimates and they were identical. I found an issue
that was somehow miraculously covered up by -random_seed 1 for 10,000
matrix solves. I was initially sent on a wild chase by noting a strong
behavior difference between DIFFERENT_NONZERO_PATTERN and
SAME_PRECONDITIONER, where DIFFERENT_NONZERO_PATTERN gave the correct
solution, but it was totally a fluke. Sorry about that.

John


On Fri, Jul 12, 2013 at 12:31 PM, Mark F. Adams <mark.adams at columbia.edu>wrote:

> >
> >
> >  if (set) {
> >    ierr = PetscRandomSetSeed(rnd,(unsigned long int)seed);CHKERRQ(ierr);
> >    ierr = PetscRandomSeed(rnd);CHKERRQ(ierr);
> >  }
> >
> > Otherwise the default seed is used, but that changes for repeat solves.
>
> OK, right I see.
>
> Anyway it is bad that the code is sensitive to the random RHS in the eigen
> estimator.  These operators are not symmetric apparently and I'm not sure
> about the nature of this non-symetry and how that can effect eigen
> estimators.  This is a fundamental disadvantage of smoothed aggregation.
>
> So at this point it is not clear if 1) there is an simple code/parameter
> change in the code that can fix this, 2) a code bug, or 3) a fundamental
> problem with cheby on these (asymmetric operators).
>
> It would be nice to verify and get some number on the differences in eigen
> estimates in these two (good and bad) solves.
>
> Also, hypre is a good robust alternative if you need to get stuff done now.
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