[Trilinos-Users] [EXTERNAL] Re: AMG Preconditioner Neumann Laplacian 32 vs 64 bit issue

Thu Dec 1 13:42:18 MST 2011

Hi Ray,

thanks for this. It is indeed very useful to know, I guess I should have
known already.

Do you see any reason why my 32bit machine gets away with the same setup?

Martin

On Thu, Dec 1, 2011 at 9:28 PM, Tuminaro, Raymond S <rstumin at sandia.gov>wrote:

>  My guess is that you have understood the issue ... but just in case it
> was not totally clear. ML's interpolation
> is exact for constants. This means that the coarsest level matrix is still
> singular. We then use a direct solver
> for this system (by default). The direct solver might do something funky
> with the singular system, and
> then ML will happily interpolate this funky thing up the hierarchy. ML
> often works with singular systems, but
> it is always a bit scary and it is best to just do an iterative solve for
> the coarsest level system.
> -Ray
>
>  ------------------------------
> *From:* Martin Stoll [martin.stoll80 at gmail.com]
> *Sent:* Thursday, December 01, 2011 12:16 PM
> *To:* Tuminaro, Raymond S
> *Cc:* Alberto F. Martín-Huertas; trilinos-users at software.sandia.gov
> *Subject:* Re: [EXTERNAL] Re: [Trilinos-Users] AMG Preconditioner Neumann
> Laplacian 32 vs 64 bit issue
>
>  Dear Ray,
>
> thanks for the comments.
>
> Yes, the Neumann Laplacian is indeed singular. In fact, from within dealII
> I simply use the ML AMG preconditioner as a black box as there are only a
> few parameters I can play with. In my case usually, type of smoother,
> smoother steps and number of cycles. So I am not really aware what is
> happening within ML and I go with all the default settings. I am sorry for
> being so ignorant but I never had any problems. Only when I moved my code
> to a different machine running 64 bits I was having problems. Again, same
> version of dealII and trilinos on my mac runs smoothly.
>
> I will look into changing some options from within deal.II to try a
> different solver for the coarse grid and hope for the best.
>
> Thanks again for the comments.
>
> Best,
> Martin
>
> 2011/12/1 Tuminaro, Raymond S <rstumin at sandia.gov>
>
>>  So, I'm guessing that the Neumann Laplacian is singular.  Are you using
>> a direct solve on the coarsest grid? Depending on how things are handled
>> here ... you might get different results. You might try switching to an
>> iterative method on the coarsest level and see what happens.
>>
>> -Ray
>>
>>  ------------------------------
>> *From:* trilinos-users-bounces at software.sandia.gov [
>> trilinos-users-bounces at software.sandia.gov] on behalf of Martin Stoll [
>> martin.stoll80 at gmail.com]
>> *Sent:* Thursday, December 01, 2011 4:29 AM
>> *To:* Alberto F. Martín-Huertas
>> *Cc:* trilinos-users at software.sandia.gov
>> *Subject:* [EXTERNAL] Re: [Trilinos-Users] AMG Preconditioner Neumann
>> Laplacian 32 vs 64 bit issue
>>
>>   Hi Alberto,
>> thanks for your comment.
>>
>> I just tried the Dirichlet Laplacian on  the velocity space, the Neumann
>> Laplacian was on the pressure space, and that worked fine.
>>
>> As I said on my laptop and the 32 bit desktop everything work but when I
>> switch to the 64 bit machine the outcome is quite different. Our software
>> engineer also tried different versions of trilinos (currently 10.4.2, which
>> is recommended by the deal.II developers). My Desktop uses trilinos 9.03
>> and my laptop 10.4.2 and they both work fine.
>>
>> Any help is greatly appreciated.
>>
>> Best,
>> Martin
>>
>> 2011/12/1 Alberto F. Martín-Huertas <amartin at cimne.upc.edu>
>>
>>>  Hi Martin,
>>>
>>>  did you observe the same behaviour with a
>>>  Dirichlet Laplacian ?
>>>
>>>  Best regards,
>>>  Alberto.
>>>
>>>  On Thu, 1 Dec 2011 12:52:08 +0100, Martin Stoll wrote:
>>> > Hi all,
>>> >
>>> > I am using deal.II and from there call the ML AMG preconditioner. I
>>> > am using it to approximate a Neumann Laplacian, which works like a
>>> > charm on
>>> > my 32 bit desktop Ubuntu machine and also on my Mac Book Pro running
>>> > Snow Leopard. Now copying this to a linux 64 bit machine I get weird
>>> > results meaning
>>> >  that applying the preconditioner results
>>> > 32 bit
>>> > norm(vin)=0.01
>>> > APPLY AMG
>>> > norm(vout)=0.15
>>> >
>>> > 64 bit
>>> > norm(vin)=0.01
>>> >  APPLY AMG
>>> >  norm(vout)=1e+11
>>> >
>>> > When instead of the Neumann Laplacian I put a mass matrix into the
>>> > AMG routine the blow does not happen.
>>> >
>>> > I am not sure where to go from here so I hope I could get some help
>>> > here.
>>> >
>>> > Best,
>>> >  Martin
>>> >
>>> > --
>>> > Martin Stoll
>>> > Postdoctoral Research Fellow
>>> >
>>> > Computational Methods in Systems and Control Theory
>>> >
>>> > Max Planck Institute for Dynamics of Complex Technical Systems
>>> > Sandtorstr. 1, D-39106 Magdeburg,Germany
>>>  > Email: martin.stoll80 at gmail.com [1]
>>> > URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/ [2]
>>> >
>>> > Tel :+49 391 6110 384
>>> >
>>> >
>>> >
>>>  > Links:
>>> > ------
>>> > [1] mailto:stollm at mpi-magdeburg.mpg.de
>>> > [2] http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/
>>>
>>> --
>>>  Alberto F. Martín-Huertas
>>>  Centre Internacional de Mètodes Numèrics a l'Enginyeria (CIMNE)
>>>  Parc Mediterrani de la Tecnologia, UPC
>>>  Esteve Terrades 5, Building C3, Office 210,
>>>  08860 Castelldefels (Barcelona, Spain)
>>>  Tel.: (+34) 9341 34223
>>>  e-mail: amartin at cimne.upc.edu
>>>
>>>  ________________
>>>  IMPORTANT NOTICE
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>>>
>>>
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>>
>>
>>
>> --
>> Martin Stoll
>> Postdoctoral Research Fellow
>>
>> Computational Methods in Systems and Control Theory
>> Max Planck Institute for Dynamics of Complex Technical Systems
>> Sandtorstr. 1, D-39106 Magdeburg,Germany
>>
>> Email: martin.stoll80 at gmail.com <stollm at mpi-magdeburg.mpg.de>
>>
>> URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/
>>
>> Tel :+49 391 6110 384
>>
>>
>
>
> --
> Martin Stoll
> Postdoctoral Research Fellow
>
> Computational Methods in Systems and Control Theory
> Max Planck Institute for Dynamics of Complex Technical Systems
> Sandtorstr. 1, D-39106 Magdeburg,Germany
> Email: martin.stoll80 at gmail.com <stollm at mpi-magdeburg.mpg.de>
>
> URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/
>
> Tel :+49 391 6110 384
>
>

-- 
Martin Stoll
Postdoctoral Research Fellow

Computational Methods in Systems and Control Theory
Max Planck Institute for Dynamics of Complex Technical Systems
Sandtorstr. 1, D-39106 Magdeburg,Germany
Email: martin.stoll80 at gmail.com <stollm at mpi-magdeburg.mpg.de>
URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/

Tel :+49 391 6110 384
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