[petsc-users] When does DIVERGED_LINE_SEARCH Happen?
Dafang Wang
dafang.wang at jhu.edu
Mon Mar 17 17:37:26 CDT 2014
Hi Peter,
My version of PETSc (v3.4.3) does not contain the bug fix you mentioned:
"+ ierr =
SNESLineSearchSetNorms(linesearch,xnorm,fnorm,ynorm);CHKERRQ(ierr);"
Would that be a problem?
I typically used the default value of -snes_stol, never setting it to
zero. I will let you know soon if you believe this is important.
Cheers,
Dafang
On 03/17/2014 06:27 PM, Peter Brune wrote:
> This may be related to a bug we had reported before to petsc-maint:
>
> https://bitbucket.org/petsc/petsc/commits/ced04f9d467b04aa83a18d3f8875c7f72c17217a
>
> What version of PETSc are you running? Also, what happens if you
> set -snes_stol to zero?
>
> Thanks,
>
> - Peter
>
>
> On Mon, Mar 17, 2014 at 5:19 PM, Dafang Wang <dafang.wang at jhu.edu
> <mailto:dafang.wang at jhu.edu>> wrote:
>
> Hi Barry,
>
> Thanks for your tips. I have read the webpage you mentioned many
> times before, but still I have been stuck on the line-search
> problem for weeks.
>
> I cannot guarantee my Jacobian is correct but I believe an
> incorrect Jacobian is very unlikely. My Jacobian-calculation code
> has been under test for a year with both analytical and realistic
> models, and the results have been good until recently when I ran a
> very realistic physical model.
>
> Also, I looked up the implementation of SNESSolve_NEWTONLS() in
> "ls.c". According to the algorithm, when the function
> "SNESLineSearchApply()" does not succeed, one may encounter two
> possible outcomes: CONVERGED_SNORM_RELATIVE (if the search step is
> too small) or otherwise, DIVERGED_LINE_SEARCH. Does this mean that
> both these two outcomes indicate that the line search fails?
>
> I ask this question because my simulation encountered many
> CONVERGED_SNORM_RELATIVE. I treated them as if my nonlinear system
> converged, accepted the nonlinear solution, and then proceeded to
> the next time step of my simulation. Apparently, such practice has
> worked well in most cases, (even when I encountered suspicious
> DIVERGED_LINE_SEARCH behaviors). However, I wonder if there are
> any potential pitfalls in my practice such as missing a nonlinear
> solve divergence and taking a partial solution as the correct
> solution.
>
> Thank you very much for your time and help.
>
> Best,
> Dafang
>
>
> On 03/15/2014 11:15 AM, Barry Smith wrote:
>
> Failed line search are almost always due to an incorrect
> Jacobian. Please let us know if the suggestions at
> http://www.mcs.anl.gov/petsc/documentation/faq.html#newton
> don't help.
>
> Barry
>
> On Mar 14, 2014, at 8:57 PM, Dafang Wang <dafang.wang at jhu.edu
> <mailto:dafang.wang at jhu.edu>> wrote:
>
> Hi,
>
> Does anyone know what the error code DIVERGED_LINE_SEARCH
> means in the SNES nonlinear solve? Or what scenario would
> lead to this error code?
>
> Running a solid mechanics simulation, I found that the
> occurrence of DIVERGED_LINE_SEARCH was very unpredictable
> and sensitive to the input values to my nonlinear system,
> although my system should not be that unstable. As shown
> by the two examples below, my system diverged in one case
> and converged in the other, although the input values in
> these two cases differed by only 1e-4,
>
> Moreover, the Newton steps in the two cases were very
> similar up to NL step 1. Since then, however, Case 1
> encountered a line-search divergence whereas Case 2
> converged successfully. This is my main confusion. (Note
> that each residual vector contains 3e04 DOF, so when their
> L2 norms differ within 1e-4, the two systems should be
> very close.)
>
> My simulation input consists of two scalar values (p1 and
> p2), each of which acts as a constant pressure boundary
> condition.
>
> Case 1, diverge:
> p1= -10.190869 p2= -2.367555
> NL step 0, |residual|_2 = 1.621402e-02
> Line search: Using full step: fnorm
> 1.621401550027e-02 gnorm 7.022558235262e-05
> NL step 1, |residual|_2 = 7.022558e-05
> Line search: Using full step: fnorm
> 7.022558235262e-05 gnorm 1.636418730611e-06
> NL step 2, |residual|_2 = 1.636419e-06
> Nonlinear solve did not converge due to
> DIVERGED_LINE_SEARCH iterations 2
> Case 2: converge:
> p1= -10.190747 p2= -2.367558
> NL step 0, |residual|_2 = 1.621380e-02
> Line search: Using full step: fnorm
> 1.621379778276e-02 gnorm 6.976373804153e-05
> NL step 1, |residual|_2 = 6.976374e-05
> Line search: Using full step: fnorm
> 6.976373804153e-05 gnorm 4.000992847275e-07
> NL step 2, |residual|_2 = 4.000993e-07
> Line search: Using full step: fnorm
> 4.000992847275e-07 gnorm 1.621646014441e-08
> NL step 3, |residual|_2 = 1.621646e-08
> Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE
> iterations 3
>
> Aside from the input values, the initial solution in both
> cases may differ very slightly. (Each case is one time
> step in a time-sequence simulation. The two cases behaved
> nearly identically up to the last time step before the
> step shown above, so their initial solutions may differ by
> a cumulative error but such error should be very small.)
> Is it possible that little difference in initial guess
> leads to different local minimum regions where the line
> search in Case 1 failed?
>
> Any comments will be greatly appreciated.
>
> Thanks,
> Dafang
> --
> Dafang Wang, Ph.D
> Postdoctoral Fellow
> Institute of Computational Medicine
> Department of Biomedical Engineering
> Johns Hopkins University
> Hackerman Hall Room 218
> Baltimore, MD, 21218
>
>
> --
> Dafang Wang, Ph.D
> Postdoctoral Fellow
> Institute of Computational Medicine
> Department of Biomedical Engineering
> Johns Hopkins University
> Hackerman Hall Room 218
> Baltimore, MD, 21218
>
>
--
Dafang Wang, Ph.D
Postdoctoral Fellow
Institute of Computational Medicine
Department of Biomedical Engineering
Johns Hopkins University
Hackerman Hall Room 218
Baltimore, MD, 21218
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