[petsc-users] SNES divergence

[Sang pham van]

Hi Barry,

When using -snes_grid_sequence <n>, how does SNES refines the mesh? Does it
detect where should the mesh be refine locally like an adaptive refinement?

Thank you.

S.


On Tue, Jun 17, 2014 at 4:09 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Jun 17, 2014, at 3:03 PM, Sang pham van <pvsang002 at gmail.com> wrote:
>
> > Hi Barry,
> >
> > I want to try the original version of Newton method, no line search nor
> trust region…
>
>    This will often fail, original version Newton just doesn’t work without
> a good initial solution.
>
> > Using grid sequencing confuses me at the point that I don't know
> explicitly the final mesh size, or even if I can estimate the mesh size
> from the input -snes_grid_sequence n it's not so convenient for me when
> some of my routines need such info from very first moment.
>
>    To do grid sequencing your code needs to be able to compute the
> function on several grids, and compute the Jacobian on several grids. If
> your code is hardwired to work on only one grid (with global variables or
> whatever) you cannot use grid sequencing and you out of luck. You should
> organize your code so that it doesn’t depend on any hardwired grid sizes.
>
>   Barry
>
>
>
> > I wish PETSc could allow us to pass in the finest mesh for grid
> sequencing.
> >
> >
> > S,
> >
> >
> > On Tue, Jun 17, 2014 at 3:46 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > On Jun 17, 2014, at 2:30 PM, Sang pham van <pvsang002 at gmail.com> wrote:
> >
> > > Thanks Barry,
> > >
> > > To refine the grid, I just put more points in the direction. For both
> fine and coarse meshes, I used simple initial guess (say constant values in
> whole domain for all variables).
> > >
> > > By using grid sequencing, is the finest mesh is the one I first input
> the solver?
> >
> >   No, you pass in the coarse one.
> > >
> > > Can you let me know what options should I use to have pure Newton
> method?
> >
> >    Not sure what you mean by pure Newton method, maybe without grid
> sequencing? It simply may not be possible to get convergence from a “poor”
> initial guess. One should always use grid sequencing if possible.
> >
> >    Barry
> >
> > >
> > > S.
> > >
> > >
> > > On Tue, Jun 17, 2014 at 3:20 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> > >
> > > On Jun 17, 2014, at 2:12 PM, Sang pham van <pvsang002 at gmail.com>
> wrote:
> > >
> > > > Hi,
> > > >
> > > > I am using DM structure and SNES to solve a 3D problem. In the
> problem I have 3 variables.
> > > >
> > > > I got SNES converged with a grid. Obtain result are physically
> right. However when I refine the grid, SNES does not always converge,
> > >
> > >    With the refined grid how are you starting the solution? Do you use
> the interpolated solution from the coarser grid (called grid sequencing) or
> just some “not good” initial guess?
> > >
> > > > the reason of divergence is line search fail or linear solver
> failed. (I also tried other type of SNES, line search seems to be the one
> best fits my problem)
> > > >
> > > > Can you please give me a suggestion to figure out the problem with
> my solver? What options should I use to have pure Newton method in SNES? Is
> there any advance option I can use with line search to improve SNES
> convergence.
> > > >
> > >
> > >    You should use grid sequencing, not only does it get convergence
> when you may not otherwise get it but it will also solve the problem
> faster. With PETSc DM you can use -snes_grid_sequence n  or
> SNESSetGridSequence() in the code to do n levels of grid sequencing.
> > >
> > >    Barry
> > >
> > > > Thank you very much.
> > > >
> > > > Minh.
> > > >
> > >
> > >
> >
> >
>
>
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