On Apr 30, 2014, at 6:46 AM, Matthew Knepley <knep...@gmail.com> wrote:
> On Wed, Apr 30, 2014 at 6:19 AM, Justin Dong <j...@rice.edu> wrote: > Thanks. If I turn on the Krylov solver, the issue still seems to persist > though. > > mpiexec -n 4 -ksp_type gmres -ksp_rtol 1.0e-13 -pc_type lu > -pc_factor_mat_solver_package superlu_dist > > I'm testing on a very small system now (24 ndofs), but if I go larger (around > 20000 ndofs) then it gets worse. > > For the small system, I exported the matrices to matlab to make sure they > were being assembled correct in parallel, and I'm certain that that they are. > > For convergence questions, always run using -ksp_monitor -ksp_view so that we > can see exactly what you run. Also run with -ksp_pc_side right > > Thanks, > > Matt > > > On Wed, Apr 30, 2014 at 5:32 AM, Matthew Knepley <knep...@gmail.com> wrote: > On Wed, Apr 30, 2014 at 3:02 AM, Justin Dong <j...@rice.edu> wrote: > I actually was able to solve my own problem...for some reason, I need to do > > PCSetType(pc, PCLU); > PCFactorSetMatSolverPackage(pc, MATSOLVERSUPERLU_DIST); > KSPSetTolerances(ksp, 1.e-15, PETSC_DEFAULT, PETSC_DEFAULT, PETSC_DEFAULT); > > 1) Before you do SetType(PCLU) the preconditioner has no type, so > FactorSetMatSolverPackage() has no effect > > 2) There is a larger issue here. Never ever ever ever code in this way. > Hardcoding a solver is crazy. The solver you > use should depend on the equation, discretization, flow regime, and > architecture. Recompiling for all those is > out of the question. You should just use > > KSPCreate() > KSPSetOperators() > KSPSetFromOptions() > KSPSolve() > > and then > > -pc_type lu -pc_factor_mat_solver_package superlu_dist > > > instead of the ordering I initially had, though I'm not really clear on what > the issue was. However, there seems to be some loss of accuracy as I increase > the number of processes. Is this expected, or can I force a lower tolerance > somehow? I am able to compare the solutions to a reference solution, and the > error increases as I increase the processes. This is the solution in > sequential: > > Yes, this is unavoidable. However, just turn on the Krylov solver > > -ksp_type gmres -ksp_rtol 1.0e-10 > > and you can get whatever residual tolerance you want. To get a specific > error, you would need > a posteriori error estimation, which you could include in a custom > convergence criterion. > > Thanks, > > Matt > > superlu_1process = [ > -6.8035811950925553e-06 > 1.6324030474375778e-04 > 5.4145340579614926e-02 > 1.6640521936281516e-04 > -1.7669374392923965e-04 > -2.8099208957838207e-04 > 5.3958133511222223e-02 > -5.4077899123806263e-02 > -5.3972905090366369e-02 > -1.9485020474821160e-04 > 5.4239813043824400e-02 > 4.4883984259948430e-04]; > > superlu_2process = [ > -6.8035811950509821e-06 > 1.6324030474371623e-04 > 5.4145340579605655e-02 > 1.6640521936281687e-04 > -1.7669374392923807e-04 > -2.8099208957839834e-04 > 5.3958133511212911e-02 > -5.4077899123796964e-02 > -5.3972905090357078e-02 > -1.9485020474824480e-04 > 5.4239813043815172e-02 > 4.4883984259953320e-04]; > > superlu_4process= [ > -6.8035811952565206e-06 > 1.6324030474386164e-04 > 5.4145340579691455e-02 > 1.6640521936278326e-04 > -1.7669374392921441e-04 > -2.8099208957829171e-04 > 5.3958133511299078e-02 > -5.4077899123883062e-02 > -5.3972905090443085e-02 > -1.9485020474806352e-04 > 5.4239813043900860e-02 > 4.4883984259921287e-04]; > > This is some finite element solution and I can compute the error of the > solution against an exact solution in the functional L2 norm: > > error with 1 process: 1.71340e-02 (accepted value) > error with 2 processes: 2.65018e-02 > error with 3 processes: 3.00164e-02 > error with 4 processes: 3.14544e-02 > > > Is there a way to remedy this? > > > On Wed, Apr 30, 2014 at 2:37 AM, Justin Dong <j...@rice.edu> wrote: > Hi, > > I'm trying to solve a linear system in parallel using SuperLU but for some > reason, it's not giving me the correct solution. I'm testing on a small > example so I can compare the sequential and parallel cases manually. I'm > absolutely sure that my routine for generating the matrix and right-hand side > in parallel is working correctly. > > Running with 1 process and PCLU gives the correct solution. Running with 2 > processes and using SUPERLU_DIST does not give the correct solution (I tried > with 1 process too but according to the superlu documentation, I would need > SUPERLU for sequential?). This is the code for solving the system: > > /* solve the system */ > KSPCreate(PETSC_COMM_WORLD, &ksp); > KSPSetOperators(ksp, Aglobal, Aglobal, DIFFERENT_NONZERO_PATTERN); > KSPSetType(ksp,KSPPREONLY); > > KSPGetPC(ksp, &pc); > > KSPSetTolerances(ksp, 1.e-13, PETSC_DEFAULT, PETSC_DEFAULT, > PETSC_DEFAULT); > PCFactorSetMatSolverPackage(pc, MATSOLVERSUPERLU_DIST); > > KSPSolve(ksp, bglobal, bglobal); > > Sincerely, > Justin > > > > > > > -- > What most experimenters take for granted before they begin their experiments > is infinitely more interesting than any results to which their experiments > lead. > -- Norbert Wiener > > > > > -- > What most experimenters take for granted before they begin their experiments > is infinitely more interesting than any results to which their experiments > lead. > -- Norbert Wiener
