[petsc-users] MATSOLVERSUPERLU_DIST not giving the correct solution

[Justin Dong]

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.


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
>
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