[petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?

[Mark F. Adams]

On Apr 17, 2013, at 4:26 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote:

> 
>> Mark F. Adams mark.adams at columbia.edu
>> Wed Apr 17 14:25:04 CDT 2013
>> 
>> 2) If you get "Indefinite PC" (I am guessing from using CG) it is because the
>> preconditioner
>>    really is indefinite (or possible non-symmetric). We improved the checking
>>    for this in one
>>    of those releases.
>> 
>> AMG does not guarantee an SPD preconditioner so why persist in trying to use
>> CG?
>> 
>> 
>> AMG is positive if everything is working correctly.
>> 
>> Are these problems only semidefinite?  Singular systems can give erratic
>> behavior.
> 
> It is a Laplace operator from Galerkin finite elements. And the PC is fine on
> ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say that the
> same PC should be positive on 4 as well.
> 
> Can you guys please CC jbakosi at lanl.gov? Thanks, J
> 

I assume that this is not a Neumann problem … you can try -pc_type gamg and -pc_type hypre.  

And PETSc is testing for essentially < 0 which is a numerically finicky thing to do.  These solvers are not bitwise identical wrt number of processors so getting different results for this test is not unreasonable.