Hi Barry, I want to try the original version of Newton method, no line search nor trust region... 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. 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 <[email protected]> wrote: > > On Jun 17, 2014, at 2:30 PM, Sang pham van <[email protected]> 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 <[email protected]> wrote: > > > > On Jun 17, 2014, at 2:12 PM, Sang pham van <[email protected]> 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. > > > > > > > > >
