[petsc-users] da_overlap and zero pivot row

[Xiangdong]

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 <bsmith at mcs.anl.gov> wrote:

>
> > On Jun 27, 2016, at 3:15 PM, Xiangdong <epscodes at gmail.com> 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
>
>
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