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 <[email protected]
<mailto:[email protected]>> 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 <[email protected]
<mailto:[email protected]>> 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
