[petsc-users] Reuse symbolic factorization with petsc - mumps

[Hong Zhang]

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