dolfin team mailing list archive

["Garth N. Wells" <g.n.wells@xxxxxxxxxx>]

It's possible to change the tolerance for zero pivots in PETSc. Have a
look at 
http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/PC/PCFactorSetZeroPivot.html#PCFactorSetZeroPivot

I made a change to the LU class a while ago so that it could deal with
matrices that have zeroes on the diagonal, such as Stokes flow with
Taylor-Hood elements.

Garth

On Thu, 2005-12-22 at 11:38 -0600, Anders Logg wrote:
> 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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> > > 
> > > _______________________________________________
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> > > 
> > 
> 
-- 
Dr. Garth N. Wells
Faculty of Civil Engineering and Geosciences
Delft University of Technology
Stevinweg 1
2628 CN Delft
The Netherlands

tel.     +31 15 278 7922
fax.     +31 15 278 6383
e-mail   g.n.wells@xxxxxxxxxx
url      http://www.mechanics.citg.tudelft.nl/~garth