Re: Zero Pivot Row in LU Factorization

[Tim Stitt <timothy.stitt@xxxxxxxx>]

Barry,

I will be using these routines from Fortran..so I am assuming that 
Fortran interfaces are available for each routine?

Also, how do I know how many sub ksp's there will be? I am assuming I 
need to dynamically allocate the subksp array in Fortran but do I know 
the size in advance? Is this related to the value 'n' ? If so, how do I 
calculate 'n'.

What is the significance of subksp[0]? Is it just the sub ksp at this 
position I should be interested in?

Finally, which of the PCFactorSetxxxxxx routines should I be using?

Sorry for the twenty questions (well nearly) but I am just a bit 
confused with this approach.

Thanks,

Tim.

Barry Smith wrote:
>   KSP *subksp;
>
>    KSPGetPC(ksp,pc)
>    PCBJacobiGetSubKSP(pc,&n,PETSC_NULL,&subksp)
>    KSPGetPC(subksp[0],&subpc);
>    PCFactorSetxxxxxx(subpc, ....
>
>   Barry
>
>
> On Nov 25, 2007, at 1:10 PM, Tim Stitt wrote:
>
> > I should also add that the code executes without this error when 
> > using 1 processor...but then displays the error when running in 
> > parallel with more than one process.
> >
> > Tim Stitt wrote:
> > > Hi all,
> > >
> > > Can anyone suggest ways of overcoming the following pivot error I 
> > > keep receiving in my PETSc code during a KSPSolve().
> > >
> > > [1]PETSC ERROR: Detected zero pivot in LU factorization
> > > see 
> > > http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#ZeroPivot! 
> > >
> > > [1]PETSC ERROR: Zero pivot row 1801 value 0.00102826 tolerance 
> > > 0.00165189 * rowsum 1.65189e+09!
> > >
> > > From checking the documentation....the error is in row 1801, which 
> > > means it is most likely not a matrix assembly issue?
> > >
> > > I tried the following prior to the solve with no luck either.....
> > >
> > > call KSPGetPC(ksp,pc,error)
> > > call PCFactorSetShiftNonzero(pc,PETSC_DECIDE,error)
> > >
> > > Is there anything else I can try?
> > >
> > > Thanks,
> > >
> > > Tim.
> >
> >
> > --Dr. Timothy Stitt <timothy_dot_stitt_at_ichec.ie>
> > HPC Application Consultant - ICHEC (www.ichec.ie)
> >
> > Dublin Institute for Advanced Studies
> > 5 Merrion Square - Dublin 2 - Ireland
> >
> > +353-1-6621333 (tel) / +353-1-6621477 (fax)


--
Dr. Timothy Stitt <timothy_dot_stitt_at_ichec.ie>
HPC Application Consultant - ICHEC (www.ichec.ie)

Dublin Institute for Advanced Studies
5 Merrion Square - Dublin 2 - Ireland

+353-1-6621333 (tel) / +353-1-6621477 (fax)