On Mon, Jun 22, 2020 at 2:33 PM Zelu Xu <janezel...@gmail.com> wrote:

> Hello Prof. Bangerth > Thank you very much for your response! > > >> Jane, >> let me take this to the deal.II mailing list, since that is the place >> where >> these kinds of questions should be asked: >> >> > I recently finished an SUPG based incompressible unsteady Navier-Stokes >> solver. >> > The test case is the flow over cylinder problem. I parallized the code. >> > Everything works well with the linear code. But when I switch to >> parallel with >> > an iterative linear solver (SolverFGMRES). It performs 10x slower than >> the >> > direct solver(SparseDirectUMFPACK) I used in the linear code. The >> iterative >> > solver typically takes 5-6 gmres iterations to solve the system. >> >> Out of curiosity, what is your preconditioner? >> > > I used the inverse of diagonal terms of K > In the auxiliary problem: (G^TK^-1G+C)delP = rhs > use diag(K)^-1 instead of K^-1; > Below is the code: I am wondering if dealii has a parallel version of > precondition_Jacobi? > > system_matrix->block(0,1).vmult (tmp1, src); // multiply with the top > right block: G > > > > //multiply with block diag(K)^-1 > > //system_matrix->block(0,0).precondition_Jacobi (tmp2, tmp1); in > serial code > > const auto &M = system_matrix->block(0, 0); > > std::pair< int, int > Mrange = M.local_range(); > > for (int i = Mrange.first; i < Mrange.second; ++i) > > tmp2(i) = tmp1(i) / M.diag_element(i); > > > > //multiply with bottom left block G^T > > system_matrix->block(1,0).vmult (tmpm, tmp2); > > > system_matrix->block(1,1).vmult (tmpc, src); // multiply with the > bottom right block: C > > > dst = tmpc; > > dst -= tmpm; > > > I tried to use the parallel direct solver SparseDirectMUMPS, but it was >> not >> > able to solve the system. >> >> What exactly happens? > > > I lost the code and was not able to reproduce the error. > I tried to reproduce the code but for a > dealii::PETScWrappers::MPI::BlockSparseMatrix, I was not able to use the > solver, which has an argument type dealii::PETScWrappers::MatrixBase. > For solver.solve(system_matrix, newton_update, system_rhs); > where system_matrix is a BlockSparseMatrix > > candidate constructor not viable: no known conversion from > > 'LA::MPI::BlockSparseMatrix' (aka > > 'dealii::PETScWrappers::MPI::BlockSparseMatrix') to 'const > > dealii::PETScWrappers::MatrixBase &' for 1st argument > > MatrixBase(const MatrixBase &) = delete; > > > I cannot remember what I did to avoid this. Do you have any suggestions? > >> >> > >> > I am wondering if this is expected? Given the test case has a system >> that is >> > small ( 2D code with 10,000 dof). And is there any other parallel >> > direct solver I can try? >> >> As a rule of thumb, direct solvers are typically faster than iterative >> solvers >> for problems with less than around 100,000 unknowns. Your problem is so >> small >> that it's not worth bothering with iterative solvers. It's probably also >> so >> small that it's not worth bothering with parallelization: parallelization >> only >> works well if you have more than around 50,000 unknowns per processor. >> You're >> still far away from that. >> >> I see. I used a very coarse mesh and I was planning on switching to > finer meshes (8x, 16x) later. > But as you point out, this may still not worth the effort. > > Best >> W. >> >> -- >> ------------------------------------------------------------------------ >> Wolfgang Bangerth email: bange...@colostate.edu >> www: http://www.math.colostate.edu/~bangerth/ >> >> -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to dealii+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/CAAAchuWgmhfV4_EyC_-p4vjg_Bzie6F8DCNbb%2BMZL9aqJmxt8Q%40mail.gmail.com.