Dear all,
Just realized that my email didn't go through because of my attachments, so
here it is:
Sorry if it took a bit long to do the runs, I wasn't feeling well yesterday.
I attached the output I get from a small problem (90elements, 621 DOFs ) with
different SNESVI types (exodusII and command line outputs). As you can see
rsaug exits with an error but ss and rs run (and their results are similar).
However, after V goes to zero at a cross section line searches for both of them
(rs,ss) fail?! Also as you can see KSP converges for every step.
These are the tolerances I pass to SNES:
user->KSP_default_rtol = 1e-12;
user->KSP_default_atol = 1e-12;
user->KSP_default_dtol = 1e3;
user->KSP_default_maxit = 50000;
user->psi_default_frtol = 1e-8; // snes_frtol
user->psi_default_fatol = 1e-8; //snes_fatol
user->psi_maxit = 500; //snes_maxit
user->psi_max_funcs = 1000; //snes_max_func_its
Ps: files are here: http://cl.ly/0Z001Z3y1k0Q0g0s2F2R
thanks,
Ata
On Jan 17, 2012, at 8:16 AM, Barry Smith wrote:
>
> Blaise,
>
> Let's not solve the problem until we know what the problem is.
> -snes_vi_monitor first then think about the cure
>
> Barry
>
> On Jan 16, 2012, at 8:49 PM, Blaise Bourdin wrote:
>
>> Hi,
>>
>> Ata and I are working together on this. The problem he describes is 1/2 of
>> the iteration of our variational fracture code.
>> In our application, E is position dependant, and typically becomes very
>> large along very thin bands with width of the order of epsilon in the
>> domain. Essentially, we expect that V will remain exactly equal to 1 almost
>> everywhere, and will transition to 0 on these bands. Of course, we are
>> interested in the limit as epsilon goes to 0.
>>
>> If the problem indeed is that it takes many steps to add the degrees of
>> freedom. Is there any way to initialize manually the list of active
>> constraints? To give you an idea, here is a link to a picture of the type of
>> solution we expect. blue=1
>> https://www.math.lsu.edu/~bourdin/377451-0000.png
>>
>> Blaise
>>
>>
>>
>>> It seems to me that the problem is that ultimately ALL of the degrees of
>>> freedom are in the active set,
>>> but they get added to it a few at a time -- and there may even be some
>>> "chatter" there -- necessitating many SNESVI steps.
>>> Could it be that the regularization makes things worse? When \epsilon \ll
>>> 1, the unconstrained solution is highly oscillatory, possibly further
>>> exacerbating the problem. It's possible that it would be better if V just
>>> diverged uniformly. Then nearly all of the degrees of freedom would bump
>>> up against the upper obstacle all at once.
>>>
>>> Dmitry.
>>>
>>> On Mon, Jan 16, 2012 at 8:05 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>
>>> What do you get with -snes_vi_monitor it could be it is taking a while to
>>> get the right active set.
>>>
>>> Barry
>>>
>>> On Jan 16, 2012, at 6:20 PM, Ataollah Mesgarnejad wrote:
>>>
>>>> Dear all,
>>>>
>>>> I'm trying to use SNESVI to solve a quadratic problem with box
>>>> constraints. My problem in FE context reads:
>>>>
>>>> (\int_{Omega} E phi_i phi_j + \alpha \epsilon dphi_i dphi_j dx) V_i -
>>>> (\int_{Omega} \alpha \frac{phi_j}{\epsilon} dx) = 0 , 0<= V <= 1
>>>>
>>>> or:
>>>>
>>>> [A]{V}-{b}={0}
>>>>
>>>> here phi is the basis function, E and \alpha are positive constants, and
>>>> \epsilon is a positive regularization parameter in order of mesh
>>>> resolution. In this problem we expect V =1 a.e. and go to zero very fast
>>>> at some places.
>>>> I'm running this on a rather small problem (<500000 DOFS) on small number
>>>> of processors (<72). I expected SNESVI to converge in couple of iterations
>>>> (<10) since my A matrix doesn't change, however I'm experiencing a slow
>>>> convergence (~50-70 iterations). I checked KSP solver for SNES and it
>>>> converges with a few iterations.
>>>>
>>>> I would appreciate any suggestions or observations to increase the
>>>> convergence speed?
>>>>
>>>> Best,
>>>> Ata
>>>
>>>
>>
>> --
>> Department of Mathematics and Center for Computation & Technology
>> Louisiana State University, Baton Rouge, LA 70803, USA
>> Tel. +1 (225) 578 1612, Fax +1 (225) 578 4276
>> http://www.math.lsu.edu/~bourdin
>>
>>
>>
>>
>>
>>
>>
>
