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 <[email protected]>
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.
<[email protected]> 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
> <[email protected]<mailto:[email protected]>> 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
> <[email protected]<mailto:[email protected]>> on
> behalf of Rahul Samala <[email protected]<mailto:[email protected]>>
> 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
