[petsc-users] MATSOLVERSUPERLU_DIST not giving the correct solution

[Barry Smith]

On Apr 30, 2014, at 6:46 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, Apr 30, 2014 at 6:19 AM, Justin Dong <jsd1 at 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 <knepley at gmail.com> wrote:
> On Wed, Apr 30, 2014 at 3:02 AM, Justin Dong <jsd1 at 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 <jsd1 at 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