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

[Hong Zhang]

On Thu, Mar 14, 2013 at 2:52 PM, Thibault Faney <tibo at berkeley.edu> wrote:
> 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 ?

Yes.
Hong
>
>
> 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
>
>