[petsc-users] KSP with MatNullSpace

[Barry Smith]

On Dec 2, 2011, at 10:57 PM, Shiyuan wrote:

> I notice that when the vector indicating the nullspace is not very accurate (norm(A*phi) very small but not zero), KSP (CG) will return KSP_INDEFINITE_PC. When b is very close to zero in AX=b, KSP (CG) has the same error. Is it controlled by some tolerance? How can I set the tolerance?  

   No it is not controlled by some tolerance, it cannot be controlled by some tolerance because the indication of indefiniteness is determined by 

     } else if ((i > 0) && (beta*betaold < 0.0)) {
       ksp->reason = KSP_DIVERGED_INDEFINITE_PC;
       ierr = PetscInfo(ksp,"diverging due to indefinite preconditioner\n");CHKERRQ(ierr);
       break;

as soon as beta*betaold is negative the entire CG algorithm breaks down and there is no way to recover.

   Barry



> Thanks. 
> 
> Shiyuan  
> 
> On Wed, Nov 23, 2011 at 5:00 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> On Nov 23, 2011, at 1:31 PM, Shiyuan wrote:
> 
> > I've check it in Matlab. the matrix A is symmetric and has no negative eigenvalues( only a zero eigen value). I've also checked that the nullspace is correct(  norm(A*phi)<1e-11);
> 
>   Hmm. You can run with -ksp_view_binary and email to petsc-maint at mcs.anl.gov the resulting file called binaryoutput.
> 
>   Barry
> 
> 
> > Thanks.
> >
> > On Wed, Nov 23, 2011 at 1:26 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >   Print out the matrix for a small grid then check in Matlab that it is symmetric and has no negative eigenvalues.
> >
> >   Barry
> >
> >
> > On Nov 23, 2011, at 9:50 AM, Shiyuan wrote:
> >
> > > Hi,
> > >     I want to solve a singular system with a known nullspace. However, I the KSP solve diverges with KSP_INDEFINTE_PC even if I disable the preconditioning by PCNONE.
> > > this is how I setup the system. What did I do wrong? Any possible causes? Thanks.
> > >
> > >    ierr=MatNullSpaceCreate(PETSC_COMM_SELF,PETSC_FALSE,1,&phi,&nsp);CHKERRV(ierr);
> > >   ierr=KSPCreate(PETSC_COMM_SELF,&ksp);CHKERRV(ierr);
> > >   ierr=KSPSetOperators(ksp,A,A,DIFFERENT_NONZERO_PATTERN);CHKERRV(ierr);
> > >   ierr=KSPSetNullSpace(ksp,nsp);CHKERRV(ierr);
> > >   ierr=KSPSetType(ksp,KSPCG);CHKERRV(ierr);
> > >   ierr=KSPGetPC(ksp,&prec);CHKERRV(ierr);
> > >   ierr=PCSetType(prec,PCNONE);CHKERRV(ierr);
> > >   ierr=KSPSetTolerances(ksp,1e-5,1e-20,1e5,10000);CHKERRV(ierr);
> > >   ierr=KSPSetFromOptions(ksp);CHKERRV(ierr);
> > >   ierr=KSPSetUp(ksp);CHKERRV(ierr);
> > >
> > > Shiyuan
> >
> >
> 
>