dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

Then I have no idea. You can specify the tolerance for the GMRES
solver in DOLFIN, but not the tolerance for checking for zero pivots
in the ILU preconditioner. This is probably in the PETSc manual
somewhere. If you find out how to make the adjustment in PETSc, then
we can add a corresponding parameter in DOLFIN.

/Anders


On Thu, Dec 22, 2005 at 06:24:33PM +0100, H. Etzel wrote:
> Like the message is telling.
> The matrix is bad conditioned, but it values are not zero if the tolerance
> is not 1e-12 but 1e-20.
> So the question is how to change the tolerance.
> 
> /Haiko
> 
> -----Ursprüngliche Nachricht-----
> Von: dolfin-dev-bounces@xxxxxxxxxx [mailto:dolfin-dev-bounces@xxxxxxxxxx] Im
> Auftrag von Anders Logg
> Gesendet: Donnerstag, 22. Dezember 2005 16:20
> An: Discussion of DOLFIN development
> Betreff: Re: [DOLFIN-dev] zero pivot
> 
> 
> Looks like you are trying to solve a singular system. Most likely,
> something is wrong with the matrix which PETSc rightly detects.
> 
> To be sure, try exporting the matrix to MATLAB/Octave and see if you
> can invert it there or compute the condition number:
> 
> File file("matrix.m");
> file << A;
> 
> Then just type
> 
>     matrix
> 
> in MATLAB/Octave and check if it looks ok.
> 
> /Anders
> 
> 
> On Thu, Dec 22, 2005 at 04:02:19PM +0100, hetzel.devel@xxxxxx wrote:
> > Hello everyone,
> > I get a problem with my solver.
> > I'm using the LU or alternative the GMRES solver, but by solving my
> System, I always get the following message from Petsc:
> > 
> > [0]PETSC ERROR: MatLUFactorNumeric_SeqAIJ() line 516 in
> src/mat/impls/aij/seq/aijfact.c
> > [0]PETSC ERROR: Detected zero pivot in LU factorization
> > see
> http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#Zer
> oPivot!
> > [0]PETSC ERROR: Zero pivot row 2950 value 4.17812e-13 tolerance 1e-12 * rs
> 0.450719!
> > [0]PETSC ERROR: MatLUFactorNumeric() line 1994 in
> src/mat/interface/matrix.c
> > [0]PETSC ERROR: PCSetUp_ILU() line 798 in
> src/ksp/pc/impls/factor/ilu/ilu.c
> > [0]PETSC ERROR: PCSetUp() line 798 in src/ksp/pc/interface/precon.c
> > [0]PETSC ERROR: KSPSetUp() line 234 in src/ksp/ksp/interface/itfunc.c
> > [0]PETSC ERROR: KSPSolve() line 332 in src/ksp/ksp/interface/itfunc.c
> > 
> > Is there an option for the solver to set the tolerance?
> > 
> > /Haiko
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> 

-- 
Anders Logg
Research Assistant Professor
Toyota Technological Institute at Chicago
http://www.tti-c.org/logg/