Hmm, this is going to be tricky to debug why it the Inf/Nan is not found
when it should be.
In a debugger you can catch/trap floating point exceptions (how to do this
depends on your debugger) and then step through the code after that to see why
PETSc KSP is not properly noting the Inf/Nan and returning. This may be
cumbersome to do if you don't know PETSc well. Is your code easy to build,
would be willing to share it to me so I can run it and debug directly? If you
know how to make docker images or something you might be able to give it to me
easily.
Barry
> On Feb 25, 2022, at 3:59 PM, Giovane Avancini <[email protected]> wrote:
>
> Mark, Matthew and Barry,
>
> Thank you all for the quick responses.
>
> Others might have a better idea, but you could run with '-info :ksp' and see
> if you see any messages like "Linear solver has created a not a number (NaN)
> as the residual norm, declaring divergence \n"
> You could also run with -log_trace and see if it is using
> KSPConvergedDefault. I'm not sure if this is the method used given your
> parameters, but I think it is.
> Mark, I ran with both options. I didn't get any messages like "linear solver
> has created a not a number..." when using -info: ksp. When turning on
> -log_trace, I could verify that it is using KSPConvergedDefault but what does
> it mean exactly? When FGMRES converges with the true residual being NaN, I
> get the following message: [0] KSPConvergedDefault(): Linear solver has
> converged. Residual norm 8.897908325511e-05 is less than relative tolerance
> 1.000000000000e-08 times initial right hand side norm 1.466597558465e+04 at
> iteration 53. No information about NaN whatsoever.
>
> We check for NaN or Inf, for example, in KSPCheckDot(). if you have the KSP
> set to error
> (https://petsc.org/main/docs/manualpages/KSP/KSPSetErrorIfNotConverged.html
> <https://petsc.org/main/docs/manualpages/KSP/KSPSetErrorIfNotConverged.html>)
> then we throw an error, but the return codes do not seem to be checked in
> your implementation. If not, then we set the flag for divergence.
> Matthew, I do not check the return code in this case because I don't want
> PETSc to stop if an error occurs during the solving step. I just want to know
> that it didn't converge and treat this error inside my code. The problem is
> that the flag for divergence is not always being set when FGMRES is not
> converging. I was just wondering why it was set during time step 921 and why
> not for time step 922 as well.
>
> Thanks for the complete report. It looks like we may be missing a check in
> our FGMRES implementation that allows the iteration to continue after a
> NaN/Inf.
>
> I will explain how we handle the checking and then attach a patch that
> you can apply to see if it resolves the problem. Whenever our KSP solvers
> compute a norm we
> check after that calculation to verify that the norm is not an Inf or Nan.
> This is an inexpensive global check across all MPI ranks because immediately
> after the norm computation all ranks that share the KSP have the same value.
> If the norm is a Inf or Nan we "short-circuit" the KSP solve and return
> immediately with an appropriate not converged code. A quick eye-ball
> inspection of the FGMRES code found a missing check.
>
> You can apply the attached patch file in the PETSC_DIR with
>
> patch -p1 < fgmres.patch
> make libs
>
> then rerun your code and see if it now handles the Inf/NaN correctly. If so
> we'll patch our release branch with the fix.
> Thank you for checking this, Barry. I applied the patch exactly the way you
> instructed, however, the problem is still happening. Is there a way to check
> if the patch was in fact applied? You can see in the attached screenshot the
> terminal information.
>
> Kind regards,
>
> Giovane
>
> Em sex., 25 de fev. de 2022 às 13:48, Barry Smith <[email protected]
> <mailto:[email protected]>> escreveu:
>
> Giovane,
>
> Thanks for the complete report. It looks like we may be missing a check
> in our FGMRES implementation that allows the iteration to continue after a
> NaN/Inf.
>
> I will explain how we handle the checking and then attach a patch that
> you can apply to see if it resolves the problem. Whenever our KSP solvers
> compute a norm we
> check after that calculation to verify that the norm is not an Inf or Nan.
> This is an inexpensive global check across all MPI ranks because immediately
> after the norm computation all ranks that share the KSP have the same value.
> If the norm is a Inf or Nan we "short-circuit" the KSP solve and return
> immediately with an appropriate not converged code. A quick eye-ball
> inspection of the FGMRES code found a missing check.
>
> You can apply the attached patch file in the PETSC_DIR with
>
> patch -p1 < fgmres.patch
> make libs
>
> then rerun your code and see if it now handles the Inf/NaN correctly. If so
> we'll patch our release branch with the fix.
>
> Barry
>
>
>
>> Giovane
>
>
>> On Feb 25, 2022, at 11:06 AM, Giovane Avancini via petsc-users
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> Dear PETSc users,
>>
>> I'm working on an inhouse code that solves the Navier-Stokes equation in a
>> Lagrangian fashion for free surface flows. Because of the large distortions
>> and pressure gradients, it is quite common to encounter some issues with
>> iterative solvers for some time steps, and because of that, I implemented a
>> function that changes the solver type based on the flag KSPConvergedReason.
>> If this flag is negative after a call to KSPSolve, I solve the same linear
>> system again using a direct method.
>>
>> The problem is that, sometimes, KSP keeps converging even though the
>> residual is NaN, and because of that, I'm not able to identify the problem
>> and change the solver, which leads to a solution vector equals to INF and
>> obviously the code ends up crashing. Is it normal to observe this kind of
>> behaviour?
>>
>> Please find attached the log produced with the options
>> -ksp_monitor_lg_residualnorm -ksp_log -ksp_view -ksp_monitor_true_residual
>> -ksp_converged_reason and the function that changes the solver. I'm
>> currently using FGMRES and BJACOBI preconditioner with LU for each block.
>> The problem still happens with ILU for example. We can see in the log file
>> that for the time step 921, the true residual is NaN and within just one
>> iteration, the solver fails and it gives the reason DIVERGED_PC_FAILED. I
>> simply changed the solver to MUMPS and it converged for that time step.
>> However, when solving time step 922 we can see that FGMRES converges while
>> the true residual is NaN. Why is that possible? I would appreciate it if
>> someone could clarify this issue to me.
>>
>> Kind regards,
>> Giovane
>>
>>
>>
>> --
>> Giovane Avancini
>> Doutorando em Engenharia de Estruturas - Escola de Engenharia de São Carlos,
>> USP
>>
>> PhD researcher in Structural Engineering - School of Engineering of São
>> Carlos. USP
>> <function.txt><log.txt>
>
>
>
> --
> Giovane Avancini
> Doutorando em Engenharia de Estruturas - Escola de Engenharia de São Carlos,
> USP
>
> PhD researcher in Structural Engineering - School of Engineering of São
> Carlos. USP
> <log.txt><patch.png>
