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

[Mark F. Adams]

Begin forwarded message:

> From: "Christon, Mark A" <christon at lanl.gov>
> Subject: Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?
> Date: April 17, 2013 7:06:11 PM EDT
> To: "Mark F. Adams" <mark.adams at columbia.edu>, "Bakosi, Jozsef" <jbakosi at lanl.gov>
> 
> Hi Mark,
> 
> Yes, looks like the new version does a little better after 2 iterations, but at the 8th iteration, the residuals increase:(
> 
> I suspect this is why PETSc is whining about an indefinite preconditioner.   
> 
> Something definitely changes as we've had about 6-8 regression tests start failing that have been running flawlessly with ML + PETSc 3.1-p8 for almost two years.
> 
> If we can understand what changed, we probably have a fighting chance of correcting it — assuming it's some solver setting for PETSc that we're not currently using.
> 
> - Mark
> 
> -- 
> Mark A. Christon
> Computational Physics Group (CCS-2)
> Computer, Computational and Statistical Sciences Division
> Los Alamos National Laboratory
> MS D413, P.O. Box 1663
> Los Alamos, NM 87545
> 
> E-mail: christon at lanl.gov
> Phone: (505) 663-5124 
> Mobile: (505) 695-5649 (voice mail)
>  
> International Journal for Numerical Methods in Fluids
> 
> From: "Mark F. Adams" <mark.adams at columbia.edu>
> Date: Wed, 17 Apr 2013 18:51:02 -0400
> To: PETSc users list <petsc-users at mcs.anl.gov>
> Cc: "Mark A. Christon" <christon at lanl.gov>
> Subject: Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?
> 
>> 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>
>> 
>> 

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