[petsc-users] A number of questions about DMDA with SNES and Quasi-Newton methods

Sat Aug 26 22:43:04 CDT 2017

Hi Barry - many thanks for taking the time to understand my many problems
and providing so much help.

The reason I was concerned that I could not alter the linesearch was when I
tried to use bt instead of the L-BFGS default, cp, the code crashed with an
error like "Could not get Jacobian".  Maybe this is an incompatibility like
you say, since L-BFGS only uses the initial Jacobian and I never tried
setting the scale type.

I took your advice and tried to shrink the problem.  First I tried
shrinking by a factor 1000 but this converged very quickly with all test
data I could provide, including the data which was problematic with the
large grid.  So I settled for a reduction in size by a factor 125.  The
grid size is 13,230.  This is a decent test case because the solver fails
to converge with the options I was using before, and it is small enough
that I can run it with the options you suggested (-snes_fd_color -snes_type
newtonls -snes_monitor -snes_linesearch_monitor -ksp_monitor -pc_type lu).

The output is 1800 lines long - how shall I share it?

On Sat, Aug 26, 2017 at 7:38 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Aug 26, 2017, at 5:56 PM, zakaryah . <zakaryah at gmail.com> wrote:
> >
> > I'm using PETSc's SNES methods to solve PDEs which result from
> Euler-Lagrange equations for the strain energy of a 3D displacement field.
> There is an additional term in the Lagrangian which describes external
> forces which arise from various data sets, and that term contains
> nonlinearities (field terms higher than linear). The grid has about 1.6e6
> elements, and the displacement field has 3 components at each grid element.
> >
> > I'm trying to solve a sequence of successively more complicated
> equations, and the latest equation is failing to converge on some data
> sets. In particular, the methods were successful for the infinitesimal bulk
> strain (compression) energy, as well as the full infinitesimal strain
> energy (bulk + shear), but I'm now trying to generalize to the finite
> strain, as certain data sets are known to result from displacement fields
> for which the infinitesimal strain is a poor approximation.
> >
> > I'm using a DMDA, closely following example 48, and my preferred solver
> is L-BFGS.
>
>    So you are using ?
>
> -snes_type qn -snes_qn_type lbfgs
>
> >
> > I have read the FAQs "Why is Newton's method not converging?" and "Why
> is my iterative linear solver not converging?" which have raised a number
> of questions:
>
>   Quasi Newton methods either don't use Jacobians or use only the initial
> Jacobian (the idea behind quasi-Newton methods is to approximate Jacobian
> information from previous iterations without having the user compute a
> Jacobian at each iteration). With PETSc's qn it only uses the Jacobian if
> you use the option
>
>    -snes_qn_scale_type Jacobian
>
> otherwise the Jacobian is never computed or used
>
>
> >
> > Is there documentation for the DMDA/SNES methods somewhere?  I don't
> understand these very well.  For example, I am not allocating any matrix
> for the global Jacobian, and I believe this prevents me from changing the
> line search.  If I'm mistaken I would love to see an example of changing
> the line search type while using DMDA/SNES.
>
>    Whether you provide a Jacobian or not is orthogonal to the line search.
>
>     You should be able to change the line search with
>
> -snes_linesearch_type bt  or nleqerr or basic or  l2  or cp
>
> not all of them may work with qn
>
>
> >
> > I don't know how to interpret the linesearch monitor.  Even for problems
> which are converging properly, the linesearch monitor reports "lssucceed=0"
> on every iteration.  Is this a problem?
>
>   It returns a 0 if the line search does not believe it has achieved
> "sufficient decrease" in the function norm (or possibly some other measure
> of decrease) you should run -snes_linesearch_monitor also with the option
> -snes_monitor to see what is happening to the function norm
>
>    For qn you can add the option
>
>  -snes_qn_monitor
>
> to get more detailed monitoring
>
>
> >
> > I'm also having trouble understanding the methods for troubleshooting.
> I suspect that I've made an error in the analytical Jacobian, which has a
> rather large number of non-zero elements, but I have no idea how to use
> -snes_type test -snes_test_display.  The FAQs mention that some
> troubleshooting tools are more useful for small test problems.  How small
> is small?
>
>    Tiny, at most a few dozen rows and columns in the Jacobin.
>
>    You should run without the -snes_test_display information, what does it
> say? Does it indicate the Jacobian or report there is likely a problem?
>
>     With DMDA you can also use -snes_fd_color to have PETSc compute the
> Jacobian for you instead of using your analytical form. If it works with
> this, but not your Jacobian then your Jacobian is wrong.
>
> >  When I try to run the program with -snes_type test -snes_test_display,
> I get errors like:
> >
> > [0]PETSC ERROR: Argument out of range [0]PETSC ERROR: Local index
> 1076396032 too large 4979879 (max) at 0
> >
> > The second size is 1 less than the number of field elements, while the
> first number seems too large for any aspect of the problem - the Jacobian
> has at most 59 non-zero columns per row.
> >
> > Because I suspect a possible error in the Jacobian, I ran with
> -snes_mf_operator -pc_type ksp -ksp_ksp_rtol 1e-12 and observed very
> similar failure to converge (diverging residual) as with the explicit
> Jacobian.
>
>    What do you get with -ksp_monitor -ksp_ksp_monitor   it sounds like the
> true Jacobian is either very ill-conditioned or your function evaluation is
> wrong.
>
> > Do I need to set an SNES method which is somehow compatible with the
> "matrix-free" approach? If I instead use -snes_mf, the problem seems to
> converge, but horrendously slowly (true residual relative decrease by about
> 1e-5 per iteration).  I suppose this supports my suspicion that the
> Jacobian is incorrect but doesn't really suggest a solution.
> >
> > Is it possible that the analytical Jacobian is correct, but somehow
> pathological, which causes the SNES to diverge?
>
>    Yes
>
> >  Neither the Jacobian nor the function have singularities.
> >
> > Thanks for any help you can provide!
>
>     Try really hard to set up a small problem (like just use a very coarse
> grid) to experiment with as you try to get convergence. Using a big problem
> for debugging convergence is a recipe for pain.
>
>    Also since you have a Jacobian I would start with -snes_fd_color
> -snes_type ls -snes_monitor -snes_linesearch_monitor -ksp_monitor -pc_type
> lu (not on a huge problem), what happens? Send the output
>
>    Barry
>
>
>
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