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

[Mark F. Adams]

> 
>  
>  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.