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

Thu Dec 1 13:28:46 MST 2011

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<mailto: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<mailto:trilinos-users-bounces at software.sandia.gov> [trilinos-users-bounces at software.sandia.gov<mailto:trilinos-users-bounces at software.sandia.gov>] on behalf of Martin Stoll [martin.stoll80 at gmail.com<mailto: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<mailto: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<mailto: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<mailto:martin.stoll80 at gmail.com> [1]
> URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/ [2]
>
> Tel :+49 391 6110 384<tel:%2B49%20391%206110%20384>
>
>
>
> Links:
> ------
> [1] mailto:stollm at mpi-magdeburg.mpg.de<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)
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 Tel.: (+34) 9341 34223<tel:%28%2B34%29%209341%2034223>
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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<mailto:stollm at mpi-magdeburg.mpg.de>

URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/

Tel :+49 391 6110 384<tel:%2B49%20391%206110%20384>

--
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<mailto:stollm at mpi-magdeburg.mpg.de>

URL: http://www.mpi-magdeburg.mpg.de/mpcsc/stollm/

Tel :+49 391 6110 384

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