[petsc-users] Using GAMG for solving singular systems

[Thomas Witkowski]

When I try to use GAMG for solviing a singular system (Laplace matrix 
with Neuman boundaries), it crashes with the following error:

[0]PETSC ERROR: --------------------- Error Message 
------------------------------------
[0]PETSC ERROR: Detected zero pivot in LU factorization:
[0]PETSC ERROR: Zero pivot row 65 value 9.19403e-17 tolerance 2.22045e-14!
[0]PETSC ERROR: MatPivotCheck_none() line 585 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/include/petsc-private/matimpl.h
[0]PETSC ERROR: MatPivotCheck() line 604 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/include/petsc-private/matimpl.h
[0]PETSC ERROR: MatLUFactorNumeric_SeqAIJ() line 570 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/mat/impls/aij/seq/aijfact.c
[0]PETSC ERROR: MatLUFactorNumeric() line 2862 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/mat/interface/matrix.c
[0]PETSC ERROR: PCSetUp_LU() line 160 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/factor/lu/lu.c
[0]PETSC ERROR: PCSetUp() line 832 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: KSPSetUp() line 278 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: PCSetUpOnBlocks_BJacobi_Singleblock() line 719 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/bjacobi/bjacobi.c
[0]PETSC ERROR: PCSetUpOnBlocks() line 865 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: KSPSetUpOnBlocks() line 154 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: KSPSolve() line 403 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: PCMGMCycle_Private() line 20 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCMGMCycle_Private() line 49 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCMGMCycle_Private() line 49 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCMGMCycle_Private() line 49 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCApplyRichardson_MG() line 104 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCApplyRichardson() line 780 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: KSPSolve_Richardson() line 69 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/ksp/impls/rich/rich.c
[0]PETSC ERROR: KSPSolve() line 446 in 
/lustre/jhome22/hdr06/hdr060/petsc/build/petsc-dev/src/ksp/ksp/interface/itfunc.c

How to fix this problem? I already checked the gamg options, but I have 
not found something what could be related to this problem.

Thomas