Re: Zero Pivot Row in LU Factorization

[Barry Smith <bsmith@xxxxxxxxxxx>]

On Dec 6, 2007, at 6:09 AM, Tim Stitt wrote:

> 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'.

   There will always be one sub ksp be default. There will only be  
more than one if you use
PCBJacobiSetLocalBlocks() or PCBJacobiSetTotalBlocks() or - 
pc_bjacobi_blocks.
In general we recommend keeping it one. This means you do not need to  
allocate
any KSP, just pass in a KSP variable

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

   This is just the first one. If you have multiply ones then you  
must loop over them, but I
recommend having just one.
> Finally, which of the PCFactorSetxxxxxx routines should I be using?

PCFactorSetZeroPivot() or PCFactorSetShiftNonzero() or  
PCFactorSetShiftPd() depending
on what you want to have happen.

   Barry

> 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)