[petsc-users] Problem of using PCFactorSetDropTolerance

[Zhang, Hong]

Sequential solver superlu ILU supports drop tolerance. For example, petsc/src/ksp/ksp/tutorials/ex2.c:
./ex2 -pc_type ilu -pc_factor_mat_solver_type superlu -help |grep superlu
...
-mat_superlu_ilu_droptol: <now 0.0001 : formerly 0.0001>: ILU_DropTol (None)

i.e., you can run it as
$ ./ex2 -pc_type ilu -pc_factor_mat_solver_type superlu -mat_superlu_ilu_droptol 0.0001 -ksp_monitor
  0 KSP Residual norm 7.483267010387e+00
  1 KSP Residual norm 5.109625265233e-05
Norm of error 5.10959e-05 iterations 1

$ ./ex2 -pc_type ilu -pc_factor_mat_solver_type superlu -mat_superlu_ilu_droptol 0.001 -ksp_monitor
  0 KSP Residual norm 7.473227154663e+00
  1 KSP Residual norm 5.916533165499e-03
  2 KSP Residual norm 1.638603182685e-06
Norm of error 1.6387e-06 iterations 2

Hong
________________________________
From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Barry Smith <bsmith at petsc.dev>
Sent: Tuesday, February 27, 2024 11:27 AM
To: Константин Мурусидзе <konstantin.murusidze at math.msu.ru>
Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] Problem of using PCFactorSetDropTolerance

We don't consider drop tolerance preconditioners as reliable or robust so I don't see anyone implementing it. On Feb 27, 2024, at 10: 11 AM, Константин Мурусидзе <konstantin. murusidze@ math. msu. ru> wrote: Thank you! Should we expect it
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   We don't consider drop tolerance preconditioners as reliable or robust so I don't see anyone implementing it.



On Feb 27, 2024, at 10:11 AM, Константин Мурусидзе <konstantin.murusidze at math.msu.ru> wrote:

Thank you! Should we expect it to appear in the near future?


27.02.2024, 18:07, "Barry Smith" <bsmith at petsc.dev>:

   I'm sorry for the confusion. PETSc does not have a drop tolerance ILU so this function does nothing as you've noted.

   Barry


On Feb 27, 2024, at 7:27 AM, Константин Мурусидзе <konstantin.murusidze at math.msu.ru<mailto:konstantin.murusidze at math.msu.ru>> wrote:

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Hello! I have such problem, while solving linear system and using PCFactorSetDropTolerance. As I understand this  function could significantly reduce number of iterations of system of solution, but there is no iterations decreasing.I had try a lot of options for this function, but there is no result
Could you suggest me if I have some mistake, while I setting up the preconditioner?

    PetscCall(KSPSetType(ksp, KSPCG));
    PetscCall(KSPSetInitialGuessNonzero(ksp,PETSC_TRUE));
    PetscCall(KSPSetNormType(ksp, KSP_NORM_DEFAULT));
    PetscCall(KSPCGSetType(ksp, KSP_CG_SYMMETRIC));

    PetscCall(KSPGetPC(ksp, &pc));
    PetscCall(PCSetType(pc, PCILU));
    PetscCall(PCFactorSetShiftType(pc, MAT_SHIFT_POSITIVE_DEFINITE));

    PetscCall(PCFactorSetLevels(pc, 2));
    PetscCall(PCFactorSetReuseOrdering(pc,PETSC_TRUE));
    PetscCall(PCFactorSetDropTolerance(pc, 1.e-5, 1.e-3, 10000));
    PetscCall(PCFactorSetZeroPivot(pc, 1.e-3));
    PetscCall(KSPSetTolerances(ksp, 1.e-8, 1.e-8, PETSC_DEFAULT, PETSC_DEFAULT));

    PetscCall(KSPSetFromOptions(ksp));

    PetscCall(KSPSolve(ksp, b, x));
     PetscCall(KSPGetIterationNumber(ksp, &its));


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