Doing the zero pivot thing is a hack and is generally a very bad idea. Looks like you should be using a PCFIELDSPLIT preconditioner likely with a -pc_fieldsplit_type schur you can start with the option -pc_fieldsplit_detect_saddle_point but in the longer run you likely should provide programmatically in the code the IS that defines the "zero block" of the matrix.
Barry > On Apr 20, 2015, at 10:03 AM, Song Gao <[email protected]> wrote: > > Dear Petsc-users, > > I modified the code based on KSP ex1f. I'm solving a 2D stream function > vorticity finite element problem. The problem is relatively small, around > 2500x2500. I choose to use single processor and LU preconditioner. But the > algorithm set some zeros on the diagonals. By checking the PCLU manual page, > I think I need shift, the code looks like this: > > call KSPGetPC(ksp,preconditioner,ierr) > call PCSetType(preconditioner,PCLU,ierr) > call PCFactorSetShiftAmount(preconditioner, 100.0D0,ierr) > call KSPSetFromOptions(ksp,ierr) > call KSPSolve(ksp, pet_rhs, pet_sol_update,ierr) > > But I still get errors here. > > [0]PETSC ERROR: --------------------- Error Message > -------------------------------------------------------------- > [0]PETSC ERROR: Zero pivot in LU factorization: > http://www.mcs.anl.gov/petsc/documentation/faq.html#ZeroPivot > [0]PETSC ERROR: Zero pivot row 4 value 0 tolerance 2.22045e-14 > [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for > trouble shooting. > [0]PETSC ERROR: Petsc Release Version 3.5.3, Jan, 31, 2015 > [0]PETSC ERROR: ./ex1f on a FEM_project named anakin by sgao Mon Apr 20 > 10:23:58 2015 > [0]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++ > --with-fc=gfortran --download-fblaslapack --download-mpich > [0]PETSC ERROR: #1 MatPivotCheck_none() line 622 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/include/petsc-private/matimpl.h > [0]PETSC ERROR: #2 MatPivotCheck() line 641 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/include/petsc-private/matimpl.h > [0]PETSC ERROR: #3 MatLUFactorNumeric_SeqAIJ() line 575 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/mat/impls/aij/seq/aijfact.c > [0]PETSC ERROR: #4 MatLUFactorNumeric() line 2893 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/mat/interface/matrix.c > [0]PETSC ERROR: #5 PCSetUp_LU() line 152 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/ksp/pc/impls/factor/lu/lu.c > [0]PETSC ERROR: #6 PCSetUp() line 902 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/ksp/pc/interface/precon.c > [0]PETSC ERROR: #7 KSPSetUp() line 306 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/ksp/ksp/interface/itfunc.c > [0]PETSC ERROR: #8 KSPSolve() line 418 in > /home/cfd/sgao/public/for_FEM/petsc-3.5.3/src/ksp/ksp/interface/itfunc.c > > > I tried other run time options, but none of them works. Some gives Zero pivot > errors, some doesn't converge. > > ./ex1f -pc_factor_shift_type NONZERO -ksp_monitor_true_residual -ksp_max_it 10 > > ./ex1f -pc_factor_shift_type NONZERO -pc_factor_shift_amount 1 > -ksp_monitor_true_residual -ksp_max_it 10 > > ./ex1f -pc_factor_shift_type POSITIVE_DEFINITE -pc_factor_shift_amount 1 > -ksp_monitor_true_residual -ksp_max_it 10 > > ./ex1f -pc_factor_nonzeros_along_diagonal -ksp_monitor_true_residual > -ksp_max_it 10 > > The matrix on a small grid is attached. Thanks > <mat.data>
