If you make the stencil width as wide at the overlap does the problem go away and you get expected convergence?
> On Jun 28, 2016, at 3:48 PM, Xiangdong <[email protected]> wrote: > > Let me illustrate my point and question by an example. I have a 1d nonlinear > problem with Nx=5 and stencil width 1. When I run it with np=2, processor 1 > own cells 1,2,3 (+ ghost 4) and processor 2 own cells 4,5 (+ ghost 3). I also > have functions FormFunctionLocal and FormJacobianLocal to compute the > residual and Jacobian. > > i) snes_type newtonls. The Jacobian is in mpiaij format distributed on two > processors. processor 1 computes the first three rows of Jac (size 3x5) and > processor 1 computes the last two rows of Jac (size 2x5). The code works fine. > > ii) snes_type nasm and da_overlap 0. Each processor has its own Jac (seqaij). > Processor 1 computes its 3x3 Jac and Processor 2 computes its 2x2 Jac. The > code works fine. > > iii) snes_type nasm and da_overlap 1. Each processor needs to computes its > own Jac. Processor 1 computes its 4x4 Jac and Processor 2 computes its 3x3 > Jac. However, for processor 1 to compute the 4-th row of its Jac, it needs > the properties information stored on cell 5, which is not available on > processor 1. The zero pivot error is from this wrong Jac for the row > corresponding to the overlap cell. The mat_fd_color does not work either for > the same reason that information from cell 5 is not available on processor 1. > > Do you have any suggestion for the overlap case? Thank you. > > Best, > Xiangdong > > On Mon, Jun 27, 2016 at 4:21 PM, Barry Smith <[email protected]> wrote: > > > On Jun 27, 2016, at 3:15 PM, Xiangdong <[email protected]> wrote: > > > > Hello everyone, > > > > I am trying different number of da_overlap to see its effects on nasm and > > aspin preconditioner. The codes works fine with -da_overlap 0. However, > > when I change the option -da_overlap 1, it crashed with the error message > > like "zero pivot row 12544 value 0 tolerance 2.2e-14". The option > > -pc_factor_nonzeros_along_diagonal does not fix this zero pivot issue. > > > > Do you have any quick suggestions for me to try to fix this issue? > > My guess is that it is bug in the da_overlap business or in the example; > normally I would not expect this to happen. Is this a PETSc example I can run > (tell me example and exact options) to reproduce the problem? > > Thanks > > Barry > > > > > Thank you. > > > > Best, > > Xiangdong > >
