How to sum up several LinearAlgebraPETSc::MPI::Vector completely_distributed_solution ?

[Phạm Ngọc Kiên]

Dear colleagues,

I am trying to solve my problem with different right_hand_sides.

For each right hand side, we can solve by the codes:

    LinearAlgebraPETSc::MPI::Vector completely_distributed_solution(locally_owned_dofs, mpi_communicator);
    SolverControl solver_control;
    PETScWrappers::SparseDirectMUMPS solver(solver_control, mpi_communicator);
    solver.solve(system_matrix, completely_distributed_solution, system_rhs);
    constraints.distribute(completely_distributed_solution);
    locally_relevant_solution = completely_distributed_solution;

I think I can write the codes to assemble the different right hand sides to solve the system with the same system matrix.

This give me the different solutions.

However, I do not know if I can sum up all the completely_distributed_solution from the different right-hand-sides in order to get a vector of solution.

Could you please tell me how to do this?

I would like to thank you very much for your support.

Best regards,

Kien

[Wolfgang Bangerth]

On 8/1/19 9:24 PM, Phạm Ngọc Kiên wrote:
>
> However, I do not know if I can sum up all the completely_distributed_solution
> from the different right-hand-sides in order to get a vector of solution.
> Could you please tell me how to do this?

You could just sum them up:

AppropriateType sum_of_solutions (...);
for (unsigned int i=0; i<...; ++i)
sum_of_solutions += solution[i];

Of course, the sum of the solutions equals the solution of a single linear
system with the sum of the right hand sides and the same matrix -- which would
by substantially cheaper to compute.

Best
W.

--
------------------------------------------------------------------------
Wolfgang Bangerth email: bang...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/

[Phạm Ngọc Kiên]

Dear Prof. Wolfgang Bangerth,

I have tried with the sum of the right hand side, but as I think, it did not work for my problem.

I will try with your guidance.

Thank you very much.

Best regards,

Kien

Vào Th 7, 3 thg 8, 2019 vào lúc 09:42 Wolfgang Bangerth <bang...@colostate.edu> đã viết:

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[Wolfgang Bangerth]

On 8/2/19 10:57 PM, Phạm Ngọc Kiên wrote:
> I have tried with the sum of the right hand side, but as I think, it did not
> work for my problem.

I think I've made this kind of point on the mailing list before: You make it
sound like you tried something, but it didn't work even though it should have,
and you just moved on and tried something else instead. That's rarely a good
approach: If it *should* work, it should work. If it doesn't, it's indicative
of a bug somewhere, and you should try to find and *understand* the bug,
rather than just go a different route that might simply result in a different
way of exposing this kind of bug. Trust me: There is great power in
*understanding* why something doesn't work!

In your case, you've chosen an alternative route that also happens to be
vastly more inefficient :-)