Hong: thank you for your answer. I can replace DIFFERENT_NONZERO_PATTERN by SAME_NONZERO_PATTERN, however this will give me the wrong answer as the zero pattern changes from a time step to another (e.g. the matrix A at time step 1 has more zeros than at time step 2) . What I can precompute is the maximal non-zero pattern. Do you mean I should compute the facorization for the maximal zero pattern before time iterations and then use SAME_NONZERO_PATTERN during the iterations ?
On Thu, Mar 14, 2013 at 3:29 PM, Hong Zhang <hzhang at mcs.anl.gov> wrote: > Tibo: > > Replace > call KSPSetOperators(KSP_A, A, A, DIFFERENT_NONZERO_PATTERN, ierr) > with > call KSPSetOperators(KSP_A, A, A, SAME_NONZERO_PATTERN, ierr) > > Run your code with option '-log_summary' to see the number of calling > MatLUSymbolic() and MatLUNumeric(). > > Hong > > > > > I am using the cray-petsc implementation on the NERSC system. > Specifically, > > I have implemented a runge kutta solver, and I need to solve a linear > system > > Ax = b several times per time step. I am using PETSC to interface with > the > > MUMPS solver for direct factorization. > > > > The matrix A is constant over a whole time step, but changes at each time > > step. What I do now is that at the beginning of each time step, I compute > > the matrix A, use the MUMPS solver to compute a LU factorization of A, > and > > then use KSPSolve to solve each linear system during the same time step. > > Here is an example of the succession of calls (inspired from example > > ex52.c): > > > > Once per time step: > > call KSPSetOperators(KSP_A, A, A, DIFFERENT_NONZERO_PATTERN, ierr) > > call KSPSetType(KSP_A, KSPPREONLY, ierr); > > call KSPGetPC(KSP_A, PC_A, ierr) > > call KSPSetTolerances(KSP_A, 1.d-20, PETSC_DEFAULT_DOUBLE_PRECISION, > > PETSC_DEFAULT_DOUBLE_PRECISION, PETSC_DEFAULT_INTEGER, ierr) > > call PCSetType(PC_A, PCLU, ierr) > > call PCFactorSetMatSolverPackage(PC_A, MATSOLVERMUMPS, ierr) > > call PCFactorSetUpMatSolverPackage(PC_A, ierr) > > call PCFactorGetMatrix(PC_A, PC_A, ierr) > > call MatMumpsSetIcntl(PC_A, 7, 5, ierr) > > call PCSetup(PC_A, ierr) > > call KSPSetUp(KSP_A, ierr) > > > > Then several times per time step: > > call KSPSolve( KSP_A, b, x, ierr) > > > > This works fine. > > However, I am able to precompute the maximal sparsity pattern of A before > > the time iterations. Therefore, I would like to use this information by > > precomputing the symbolic factorization before the time iterations start, > > and only compute the numerical factorization at each time step. Is there > a > > way to use MUMPS in PETSC to do this ? I would appreciate any > help/reference > > on this topic. > > > > Thank you, > > > > Tibo > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130314/fabc8fa3/attachment.html>
