On Fri, May 24, 2013 at 2:55 PM, Jed Brown <[email protected]> wrote:
> Michele Rosso <[email protected]> writes: > > > Hi Jed, > > > > I followed your suggestion by using: > > > > -pc_type gamg -pc_mg_cycle_type v -pc_gamg_agg_nsmooths 1 > > > > This works perfectly if I have a non-singular matrix. When instead I use > > periodic conditions for my system ( I set the nullspace removal > > correctly ), > > I receive an error saying a zero pivot is detected in the LU > > factorization. So, after some research, I found in the mailinglist a fix > : > > > > -pc_type gamg -pc_mg_cycle_type v -pc_gamg_agg_nsmooths 1 > > -mg_coarse_pc_factor_shift_nonzero > > It'll need to be -mg_coarse_sub_pc_factor_shift_nonzero > > With petsc-3.4 (which you should upgrade to), use > -mg_coarse_sub_pc_factor_shift_type NONZERO > > The reason you need this "sub" prefix is that the code always restricts > using block Jacobi (usually localized so that all the entries are in one > block), before applying the direct coarse solver. I think this is less elegant than -mg_coarse_pc_type svd Matt > > Still I am receiving the following error > > > > > > [0]PETSC ERROR: --------------------- Error Message > > ------------------------------------ > > [0]PETSC ERROR: Detected zero pivot in LU factorization: > > see http://www.mcs.anl.gov/petsc/documentation/faq.html#ZeroPivot! > > [0]PETSC ERROR: Zero pivot row 280 value 6.5908e-17 tolerance > 2.22045e-14! > > [0]PETSC ERROR: > > ------------------------------------------------------------------------ > > [0]PETSC ERROR: Petsc Release Version 3.3.0, Patch 3, Wed Aug 29 > > 11:26:24 CDT 2012 > > [0]PETSC ERROR: See docs/changes/index.html for recent updates. > > [0]PETSC ERROR: See docs/faq.html for hints about trouble shooting. > > [0]PETSC ERROR: See docs/index.html for manual pages. > > [0]PETSC ERROR: > > ------------------------------------------------------------------------ > > [0]PETSC ERROR: ./hit on a named nid09458 by Unknown Fri May 24 > > 14:40:48 2013 > > [0]PETSC ERROR: Libraries linked from > > [0]PETSC ERROR: Configure run at > > [0]PETSC ERROR: Configure options > > [0]PETSC ERROR: > > ------------------------------------------------------------------------ > > [0]PETSC ERROR: MatPivotCheck_none() line 583 in > > > src/mat/impls/aij/seq//ptmp/skelly/petsc/3.3.03/cray_interlagos_build/real/src/include/petsc-private/matimpl.h > > [0]PETSC ERROR: MatPivotCheck() line 602 in > > > src/mat/impls/aij/seq//ptmp/skelly/petsc/3.3.03/cray_interlagos_build/real/src/include/petsc-private/matimpl.h > > [0]PETSC ERROR: MatLUFactorNumeric_SeqAIJ() line 585 in > > src/mat/impls/aij/seq/aijfact.c > > [0]PETSC ERROR: MatLUFactorNumeric() line 2803 in > src/mat/interface/matrix.c > > [0]PETSC ERROR: PCSetUp_LU() line 160 in src/ksp/pc/impls/factor/lu/lu.c > > [0]PETSC ERROR: PCSetUp() line 832 in src/ksp/pc/interface/precon.c > > [0]PETSC ERROR: KSPSetUp() line 278 in src/ksp/ksp/interface/itfunc.c > > [0]PETSC ERROR: PCSetUpOnBlocks_BJacobi_Singleblock() line 715 in > > src/ksp/pc/impls/bjacobi/bjacobi.c > > [0]PETSC ERROR: PCSetUpOnBlocks() line 865 in > src/ksp/pc/interface/precon.c > > [0]PETSC ERROR: KSPSetUpOnBlocks() line 154 in > > src/ksp/ksp/interface/itfunc.c > > [0]PETSC ERROR: KSPSolve() line 403 in src/ksp/ksp/interface/itfunc.c > > [0]PETSC ERROR: PCMGMCycle_Private() line 20 in src/ksp/pc/impls/mg/mg.c > > [0]PETSC ERROR: PCMGMCycle_Private() line 49 in src/ksp/pc/impls/mg/mg.c > > [0]PETSC ERROR: PCMGMCycle_Private() line 49 in src/ksp/pc/impls/mg/mg.c > > [0]PETSC ERROR: PCMGMCycle_Private() line 49 in src/ksp/pc/impls/mg/mg.c > > [0]PETSC ERROR: PCApply_MG() line 326 in src/ksp/pc/impls/mg/mg.c > > [0]PETSC ERROR: PCApply() line 384 in src/ksp/pc/interface/precon.c > > [0]PETSC ERROR: KSPSolve_CG() line 139 in src/ksp/ksp/impls/cg/cg.c > > [0]PETSC ERROR: KSPSolve() line 446 in src/ksp/ksp/interface/itfunc.c > > > > What could the reason be? > > Thank you, > > > > Michele > > > > > > > > On 05/17/2013 07:35 PM, Michele Rosso wrote: > >> Thank you very much. I will try and let you know. > >> > >> Michele > >> > >> On 05/17/2013 07:01 PM, Jed Brown wrote: > >>> Michele Rosso<[email protected]> writes: > >>> > >>>> I noticed that the problem appears even if I use CG with the default > >>>> preconditioner: commenting KSPSetDM() solves the problem. > >>> Okay, this issue can't show up if you use SNES, but it's a consequence > >>> of making geometric multigrid work with a pure KSP interface. You can > >>> either use KSPSetComputeOperators() to put your assembly in a function > >>> (which will also be called on coarse levels if you use -pc_type mg > >>> without Galerkin coarse operators) or you can can provide the Jacobian > >>> using KSPSetOperators() as usual, but also call KSPSetDMActive() so > that > >>> the DM is not used for computing/updating the Jacobian. > >>> > >>> The logic is cleaner in petsc-3.4 and I think it just does the right > >>> thing in your case. > >>> > >>>> So basically without a proper grid (it seems no grid with an even > >>>> numbers of nodes qualifies) and with my own system matrix, I cannot > use > >>>> any type of multigrid > >>>> pre-conditioner? > >>> You can use all the AMG methods without setting a DM. > >>> > >> > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
