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

[Mark F. Adams]

I see you are using icc.  Perhaps our icc changed a bit between versions.  These results look like both solves are working and the old does a little better (after two iterations).

Try using jacobi instead of icc.


On Apr 17, 2013, at 6:21 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote:

>> On 04.17.2013 15:38, Matthew Knepley wrote:
>> 
>>> On 04.17.2013 14:26, Jozsef Bakosi 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.
>> 
>> Why is it safe? Because it sounds plausible? Mathematics is replete with things
>> that sound plausible and are false. Are there proofs that suggest this? Is there
>> computational evidence? Why would I believe you?
> 
> Okay, so here is some additional information:
> 
> I tried both old and new PETSc versions again, but now only taking 2 iterations
> (both with 4 CPUs) and checked the residuals. I get the same exact PC from ML in
> both cases, however, the residuals are different after both iterations:
> 
> Please do a diff on the attached files and you can verify that the ML
> diagnostics are exactly the same: same max eigenvalues, nodes aggregated, etc,
> while the norm coming out of the solver at the end at both iterations are
> different.
> 
> We reproduced the same exact behavior on two different linux platforms.
> 
> Once again: same application source code, same ML source code, different PETSc:
> 3.1-p8 vs. 3.3-p6.
> <old.out><new.out>