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? 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 > > > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20111202/5316350e/attachment-0001.htm>
