Re: Zero Pivot Row in LU Factorization

[Tim Stitt <timothy.stitt@xxxxxxxx>]

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.

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)