Zero Pivot Row in LU Factorization

[Matthew Knepley]

On Dec 7, 2007 9:58 AM, Tim Stitt <timothy.stitt at ichec.ie> wrote:
> Barry,
>
> I added the following lines to my Fortran code:
>
> call KSPGetPC(ksp,pc,error)
> call KSPSetUp(ksp,error)
> oneInt=1
> call PCBJacobiGetSubKSP(pc,oneInt,PETSC_NULL,kspSub,error)
> call KSPGetPC(kspSub,pcSub,error)
> call PCFactorSetShiftNonzero(pcSub,PETSC_DECIDE,error)
>
> Now the parallel code goes beyond the zero pivot problem I was getting
> in the KSPSolve()...but only process 0 seems to complete the KSPSolve()
> and Process 1 and higher never makes it out of the KSPSolve() i.e.
> process 0 moves on and performs post-KSPSolve work (just some print
> statements) while the other processes never get out of KSPSolve(). My
> job only terminates once the requested wallclock expires. Again when
> running with only 1 process everything terminates successfully.

This does not seem possible. BJacobi synchronizes at each step for a
residual evaluation. Are you sure you did not call KSPSolve() on the
inner KSP?

  Matt

> Any ideas? Have I done something stupid with the instructions above?
>
> Thanks,
>
> Tim.
>
> Barry Smith wrote:
> >
> > 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)
> >>
> >
>
>
> --
> 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)
>
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which
their experiments lead.
-- Norbert Wiener