On Fri, Jul 26, 2013 at 2:32 PM, Matthew Knepley <[email protected]> wrote:
> On Fri, Jul 26, 2013 at 7:28 AM, Bishesh Khanal <[email protected]>wrote: > >> >> >> >> 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: >> > > You want Algebraic Multigrid > > -pc_type fieldsplit -pc_fieldsplit_detect_saddle_point > -pc_fieldsplit_type schur > -fieldsplit_0_pc_type gamg > -fieldsplit_1_ksp_type fgmres > -fieldsplit_1_ksp_constant_null_space > -fieldsplit_1_ksp_monitor_short > -fieldsplit_1_pc_type lsc > -ksp_converged_reason > > I tried the above set of options but the solution I get seem to be not correct. The velocity field I get are quite different than the one I got before without using gamg which were the expected one. Note: (Also, I had to add one extra option of -fieldsplit_1_ksp_gmres_restart 100 , because the fieldsplit_1_ksp residual norm did not converge within default 30 iterations before restarting). > Matt > > >> 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 >> >> >> > > > -- > 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 >
