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
> >
> >
>
>
