On Wed, Jul 17, 2013 at 9:48 PM, Jed Brown <[email protected]> wrote:
> Bishesh Khanal <[email protected]> writes: > > > Now, I implemented two different approaches, each for both 2D and 3D, in > > MATLAB. It works for the smaller sizes but I have problems solving it for > > the problem size I need (250^3 grid size). > > I use staggered grid with p on cell centers, and components of v on cell > > faces. Similar split up of K to cell center and faces to account for the > > variable viscosity case) > > Okay, you're using a staggered-grid finite difference discretization of > variable-viscosity Stokes. This is a common problem and I recommend > starting with PCFieldSplit with Schur complement reduction (make that > work first, then switch to block preconditioner). Ok, I made my 3D problem work with PCFieldSplit with Schur complement reduction using the options: -pc_fieldsplit_type schur -pc_fieldsplit_detect_saddle_point -fieldsplit_1_ksp_constant_null_space You can use PCLSC or > (probably better for you), assemble a preconditioning matrix containing > the inverse viscosity in the pressure-pressure block. This diagonal > matrix is a spectrally equivalent (or nearly so, depending on > discretization) approximation of the Schur complement. The velocity > block can be solved with algebraic multigrid. Read the PCFieldSplit > docs (follow papers as appropriate) and let us know if you get stuck. > Now, I got a little confused in how exactly to use command line options to use multigrid for the velocity bock and PCLS for the pressure block. After going through the manual I tried the following: -pc_type fieldsplit -pc_fieldsplit_type schur -pc_fieldsplit_detect_saddle_point -fieldsplit_0_pc_type mg -fieldsplit_0_pc_mg_levels 2 -fieldsplit_0_pc_mg_galerkin -fieldsplit_1_ksp_type fgmres -fieldsplit_1_ksp_constant_null_space -fieldsplit_1_ksp_monitor_short -fieldsplit_1_pc_type lsc -ksp_converged_reason but I get the following errror: [0]PETSC ERROR: --------------------- Error Message ------------------------------------ [0]PETSC ERROR: Null argument, when expecting valid pointer! [0]PETSC ERROR: Null Object: Parameter # 2! [0]PETSC ERROR: ------------------------------------------------------------------------ [0]PETSC ERROR: Petsc Release Version 3.4.2, Jul, 02, 2013 [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: src/AdLemMain on a arch-linux2-cxx-debug named edwards by bkhanal Fri Jul 26 14:23:40 2013 [0]PETSC ERROR: Libraries linked from /home/bkhanal/Documents/softwares/petsc-3.4.2/arch-linux2-cxx-debug/lib [0]PETSC ERROR: Configure run at Fri Jul 19 14:25:01 2013 [0]PETSC ERROR: Configure options --with-cc=gcc --with-fc=g77 --with-cxx=g++ --download-f-blas-lapack=1 --download-mpich=1 -with-clanguage=cxx --download-hypre=1 [0]PETSC ERROR: ------------------------------------------------------------------------ [0]PETSC ERROR: MatPtAP() line 8166 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/mat/interface/matrix.c [0]PETSC ERROR: PCSetUp_MG() line 628 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/impls/mg/mg.c [0]PETSC ERROR: PCSetUp() line 890 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/interface/precon.c [0]PETSC ERROR: KSPSetUp() line 278 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c [0]PETSC ERROR: KSPSolve() line 399 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c [0]PETSC ERROR: PCApply_FieldSplit_Schur() line 807 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/impls/fieldsplit/fieldsplit.c [0]PETSC ERROR: PCApply() line 442 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/interface/precon.c [0]PETSC ERROR: KSP_PCApply() line 227 in /home/bkhanal/Documents/softwares/petsc-3.4.2/include/petsc-private/kspimpl.h [0]PETSC ERROR: KSPInitialResidual() line 64 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itres.c [0]PETSC ERROR: KSPSolve_GMRES() line 239 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/impls/gmres/gmres.c [0]PETSC ERROR: KSPSolve() line 441 in /home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c
