[petsc-users] Problem with SNES convergence

[Ling Zou]

Rahul, please see the logic I used to reduce time step size.

Hope this helps you.


Ling


for (timestep = 1; timestep <= N; timestep++) // loop over time steps
{
  // before trying to solve a time step, 1) it is still not successful; 2) we are not giving up; 3) haven't failed yet
  bool give_up = false; bool success = false; bool experienced_fail_this_time_step = false;
  // save solutions and old solutions into u_copy and u_old_copy, in case a time step fails and we restart from this saved solutions
  VecCopy(u, u_copy); VecCopy(u_old, u_old_copy);

  while ((!give_up) && (!success)) // as long as not successful and not giving up, we solve again with smaller time step
  {
    if (time_step_size < dt_min) { give_up = true; break; } // ok, bad luck, give up due to time step size smaller than a preset value
    if (experienced_fail_this_time_step) { // get the vectors from backups if this is a re-try, i.e., already failed with a larger time step
      VecCopy(u_old_copy, u); VecCopy(u_old_copy, u_old);
    }

    try {
      SNESSolve(snes, NULL, u);
      SNESGetConvergedReason(snes, &snes_converged_reason);

      if (snes_converged_reason > 0)  success = true; // yes, snes converged
      else { // no, snes did not converge
        cutTimeStepSize(); // e.g., dt / 2
        experienced_fail_this_time_step = true;
      }
    }
    catch (int err) { // in case your own pieces of code throws an exception
      std::cout << "An exception occurred." << std::endl;
      success = false;
      cutTimeStepSize(); // e.g., dt / 2
      experienced_fail_this_time_step = true;
    }
  }

  if (success) {
    // output, print, whatever
    // duplicate current solution to old solution in preparing next time step
    VecCopy(u, u_old);
    // you can increase time step size here, e.g. * 2
    increaseTimeStepSize();
  }

  if (give_up) {
    simulationFailed = true;
    std::cerr << "Simulation failed.\n";
    //exit(1);// dont exit(1) now, just break the for-loop, let PETSc clean its workspace.
    break;
  }
}


________________________________
From: Rahul Samala <srahul_05 at yahoo.co.in>
Sent: Wednesday, August 29, 2018 10:37:30 PM
To: Ling Zou; Smith, Barry F.
Cc: PETSc Users List
Subject: Re: [petsc-users] Problem with SNES convergence

Thank you Ling, I would definitely like to look at your code for reducing timestep size.
Thank you Barry for your inputs.

--
Rahul.

On Wednesday, August 29, 2018, 9:02:00 PM GMT+5:30, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:



Current time (before start of timestep) 52.5048,    iter=5380     Timestep=864.000000
  0 SNES Function norm 1.650467412595e+05
    0 KSP preconditioned resid norm 3.979123221160e+03 true resid norm 1.650467412595e+05 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 9.178246525982e-11 true resid norm 7.006473307032e-09 ||r(i)||/||b|| 4.245144892632e-14
  Linear solve converged due to CONVERGED_RTOL iterations 1
  1 SNES Function norm 6.722712947273e+02
  Linear solve did not converge due to DIVERGED_NANORINF iterations 0
Nonlinear solve did not converge due to DIVERGED_LINEAR_SOLVE iterations 1

    This usually is an indicator that the LU (ILU) factorization has hit a zero pivot (hence the linear solve has a divide by zero so gives the DIVERGED_NANORINF flag).

    You can/should call SNESGetConvergedReason() immediately after each SNESSolve(), if the result is negative that means something has failed in the nonlinear solve and you can try cutting the time-step and trying again.

    Good luck,

    Barry


> On Aug 29, 2018, at 10:11 AM, Ling Zou <Ling.Zou at inl.gov<mailto:Ling.Zou at inl.gov>> wrote:
>
> 1) My experience is that this kind of bug or sudden death (everything is fine till suddenly something is broken) is very difficult to debug/fix. I looked at your txt files and could not give any quick comments. Maybe PETSc developers have better idea on this.
> 2) I do have successful experience on reducing time step size when PETSc fails solving or my own piece of code throws an exception. If you are interested, I can share them.
>
> -Ling
> From: petsc-users <petsc-users-bounces at mcs.anl.gov<mailto:petsc-users-bounces at mcs.anl.gov>> on behalf of Rahul Samala <srahul_05 at yahoo.co.in<mailto:srahul_05 at yahoo.co.in>>
> Sent: Wednesday, August 29, 2018 8:36:58 AM
> To: PETSc Users List
> Subject: [petsc-users] Problem with SNES convergence
>
> Hello PetSc users,
>
> 1)  I have problem with SNES convergence. I call SNESSolve  in a time loop and use the inbuilt Jacobian feature. The code works fine for about 5380 time steps after which it breaks down. The solution till that point looks fine. I have used newtonls of type l2. (newtontr and others aren't working). Since I have used inbuilt Jacobian feature and the code worked for about 5000 time steps I don't understand the reason for failure, is it an incorrect function evaluation?  Attached are the outputs with -pc_type lu and ilu along with -snes_linesearch_type l2 -snes_converged_reason -snes_monitor -snes_view -ksp_converged_reason -ksp_monitor_true_residual
>
> 2)  How to get hold of failure signal, like Nonlinear solve DIVERGED_MAX_IT or DIVERGED_LINEAR_SOLVE so that whenever it occurs I can use a reduced time step and see if the code converges.
>
> Thank you,
> Rahul.
>
> output_ilu.txt
>
>
> output_ilu.txt
>
>
>
> output_lu.txt
>
>
> output_lu.txt
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