[petsc-users] SNES line search

[Dmitry Karpeev]

On Wed, Oct 17, 2012 at 2:11 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Wed, Oct 17, 2012 at 2:02 PM, Shiyuan Gu <sgu at anl.gov> wrote:
>
>> Hi,
>>     I am using petsc-3.3-p2.  For my particular equation, for the first
>> few iterations, the residual norm is effectively reduced and the solution X
>> is very close to the exact solution (the residual norm is e-17). Then in
>> the next iteration, SNES_KSPSolve(....) returns a newton step very close to
>> zero which is correct, but the line search after it fails, and the function
>> SNESLSCheckLocalMin_Private(...) is called. In the function
>> SNESLSCheckLocalMin_Private(),
>> the solution vector X is rewritten to X=J^T*F where J is the Jacobian and
>> F is the residual. Since F is very close to zero, X is also zero which is
>> wrong.
>> Is there a way to skip the line search?(if the line search is not
>> performed, there would be no problem for this particular equation).  How
>> should we handle this situation?
>>
>
> You can turn off the line search with -snes_linesearch_type basic, but I
> recommend setting an appropriate atol and stol instead, to prevent
> attempting to solve the system more accurately than machine precision when
> given a good initial guess.
>
Maybe it wouldn't be a bad idea to (a) not use the solution vector as a
temporary in detecting the local minimum, (b) print the norm of the last
solution before bailing out of the Newton loop?
Dmitry.

>
>
>> Also, after the solution vector X is rewritten by
>> SNESLSCheckLocalMin_Private(), the new residual  norm is not printed on
>> screen (which is huge for this particular problem).  Since the residual
>> norm printed on the screen is going to machine accuracy,  users might think
>> that the SNESSolve converges nicely and the returned solution is correct.
>> This seems a bit misleading in my opinion.
>>
>
> Are you checking -snes_converged_reason?
>
>
>>
>> Thanks.
>>
>> Shiyuan
>>
>>
>
>
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