<html><head></head><body><div style="font-family:garamond, new york, times, serif;font-size:13px;"><div>Thank you Ling, I would definitely like to look at your code for reducing timestep size.</div><div>Thank you Barry for your inputs.</div><div><br></div><div>--</div><div>Rahul.<br></div><div><br></div><div id="yahoo_quoted_6410304017" class="yahoo_quoted">
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On Wednesday, August 29, 2018, 9:02:00 PM GMT+5:30, Smith, Barry F. <bsmith@mcs.anl.gov> wrote:
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<div><div dir="ltr"><br clear="none">Current time (before start of timestep) 52.5048, iter=5380 Timestep=864.000000 <br clear="none"> 0 SNES Function norm 1.650467412595e+05 <br clear="none"> 0 KSP preconditioned resid norm 3.979123221160e+03 true resid norm 1.650467412595e+05 ||r(i)||/||b|| 1.000000000000e+00<br clear="none"> 1 KSP preconditioned resid norm 9.178246525982e-11 true resid norm 7.006473307032e-09 ||r(i)||/||b|| 4.245144892632e-14<br clear="none"> Linear solve converged due to CONVERGED_RTOL iterations 1<br clear="none"> 1 SNES Function norm 6.722712947273e+02 <br clear="none"> Linear solve did not converge due to DIVERGED_NANORINF iterations 0<br clear="none">Nonlinear solve did not converge due to DIVERGED_LINEAR_SOLVE iterations 1<br clear="none"><br clear="none"> 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). <br clear="none"><br clear="none"> 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.<br clear="none"><br clear="none"> Good luck,<br clear="none"><br clear="none"> Barry<br clear="none"><br clear="none"><div class="yqt1765626742" id="yqtfd14853"><br clear="none">> On Aug 29, 2018, at 10:11 AM, Ling Zou <<a shape="rect" ymailto="mailto:Ling.Zou@inl.gov" href="mailto:Ling.Zou@inl.gov">Ling.Zou@inl.gov</a>> wrote:<br clear="none">> <br clear="none">> 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.<br clear="none">> 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.<br clear="none">> <br clear="none">> -Ling<br clear="none">> From: petsc-users <<a shape="rect" ymailto="mailto:petsc-users-bounces@mcs.anl.gov" href="mailto:petsc-users-bounces@mcs.anl.gov">petsc-users-bounces@mcs.anl.gov</a>> on behalf of Rahul Samala <<a shape="rect" ymailto="mailto:srahul_05@yahoo.co.in" href="mailto:srahul_05@yahoo.co.in">srahul_05@yahoo.co.in</a>><br clear="none">> Sent: Wednesday, August 29, 2018 8:36:58 AM<br clear="none">> To: PETSc Users List<br clear="none">> Subject: [petsc-users] Problem with SNES convergence<br clear="none">> <br clear="none">> Hello PetSc users,<br clear="none">> <br clear="none">> 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<br clear="none">> <br clear="none">> 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.<br clear="none">> <br clear="none">> Thank you,<br clear="none">> Rahul.<br clear="none">> <br clear="none">> output_ilu.txt<br clear="none">> <br clear="none">> <br clear="none">> output_ilu.txt<br clear="none">> <br clear="none">> <br clear="none">> <br clear="none">> output_lu.txt<br clear="none">> <br clear="none">> <br clear="none">> output_lu.txt<br clear="none"></div></div></div>
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