Yes. thanks,.
Shao-Ching On Thu, Aug 07, 2008 at 09:37:38PM +0800, berry wrote: > Thanks... > > Shao-Ching, have you solved this problem? > > > On Mon, Aug 4, 2008 at 11:49 AM, Barry Smith <bsmith at mcs.anl.gov> wrote: > > > > > One reason not to use this approach is that it can result in a very > > ill-conditioned linear > > system that may be difficult for an iterative solver. > > > > Barry > > > > > > On Aug 3, 2008, at 10:09 PM, berry wrote: > > > > hi, shao-ching > >> > >> Why not try to change the singular matrix to be non-singular ? > >> I mean set the last column and row of the matrix to be zero except > >> diagonal element( which is set to 1). > >> > >> > >> > >> On Mon, Aug 4, 2008 at 9:23 AM, Shao-Ching Huang <schuang at ats.ucla.edu> > >> wrote: > >> Hi Barry: > >> > >> Thanks you for your suggestions. Now both sequential and parallel > >> cases work when I use CG. > >> > >> Shao-Ching > >> > >> > >> On Fri, Aug 01, 2008 at 08:56:23PM -0500, Barry Smith wrote: > >> > > >> > It is not the null space that is the problem. > >> > > >> > You are right to try the shift but > >> > 1) first run on one process using -pc_type icc -ksp_type cg - > >> > pc_factor_shift_positive_definite > >> > this will keep increasing the shift until it produces a positive- > >> > definite preconditioner. > >> > > >> > Run also with -ksp_view to confirm that it is using all the options you > >> > provided. (you can run with -help to > >> > see the option names) > >> > > >> > 2) If you have the sequential converging then use -pc_type bjacobi - > >> > sub_pc_type icc -pc_factor_shift_positive_definite > >> > > >> > Barry > >> > > >> > On Aug 1, 2008, at 7:52 PM, Shao-Ching Huang wrote: > >> > > >> >> Hi, > >> >> > >> >> I am trying to use CG to solve a singular systems (i.e. Poisson > >> >> equation with periodic conditions on all boundaries). > >> >> > >> >> The code works when I use GMRES, but it diverges when I switch CG. > >> >> > >> >> Since the null space is a constant vector, in the code I have: > >> >> > >> >> KSP ksp; > >> >> MatNullSpace nullspace; > >> >> ... > >> >> MatNullSpaceCreate(MPI_COMM_WORLD, PETSC_TRUE, 0, PETSC_NULL, > >> >> &nullspace); > >> >> KSPSetNullSpace(ksp, nullspace); > >> >> MatNullSpaceRemove(nullspace, f->p_rhs, PETSC_NULL); > >> >> > >> >> KSPSetFromOptions(ksp); > >> >> KSPSetUp(ksp); > >> >> KSPSolve(ksp, f->p_rhs, f->phi); > >> >> > >> >> [f->p_rhs is the RHS vector. f->phi is the solution vector.] > >> >> > >> >> When I use "-ksp_type gmres", it converges (shown by -ksp_monitor). > >> >> > >> >> However, when I switch to "-ksp_type cg", it diverges. > >> >> In this case (cg), "-ksp_converged_reason" says: > >> >> > >> >> Linear solve did not converge due to DIVERGED_INDEFINITE_PC > >> >> iterations 1 > >> >> > >> >> I also try adding "-sub_pc_factor_shift_nonzero 0.0000000001" but the > >> >> CG case still fails. > >> >> > >> >> Am I missing some step in handling the null space when using CG? > >> >> > >> >> Thanks, > >> >> > >> >> Shao-Ching Huang > >> >> > >> >> > >> > >> > >> > >> > >> -- > >> Pang Shengyong > >> Solidification Simulation Lab, > >> State Key Lab of Mould & Die Technology, > >> Huazhong Univ. of Sci. & Tech. China > >> > > > >
