[petsc-users] SNES line search

[Dmitry Karpeev]

On Wed, Oct 17, 2012 at 2:56 PM, Peter Brune <prbrune at gmail.com> wrote:

>
>
> On Wed, Oct 17, 2012 at 2:40 PM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>
>>
>>
>> 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,
>>
>
> Dumping garbage into X upon failure appears to be my careless mistake.
> I'll fix it in 3.3.
>
>
>>
>>
>>
> (b) print the norm of the last solution before bailing out of the Newton
>> loop?
>>
>
> This can go into the -info output.
>
Why not (also) let SNESMonitor() be called on that last solution?
Dmitry.

>
>
> - Peter
>
>
>>  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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