Matthew Knepley <[email protected]> writes: > On Tue, Jan 5, 2021 at 9:52 PM Barry Smith <[email protected]> wrote: > >> >> Ah, -snes_fd_color so it was already using finite differencing with >> coloring to compute the Jacobian which explains why the differences below >> are exactly zero. >> >> Implicit time-step schemes essentially add terms like I/dt to the >> Jacobian evaluation (and the function defining the ODE) so for tiny >> time-steps the nonlinear system gets easier and easier to solve (the >> nonlinear function becomes linear) But we didn't see that with your earlier >> run where dt 3.72529e-13 (which is absurdly small). for tiny time-steps >> SNES still made no progress. It is hard to understand how this is possible, >> regardless of the problem you are solving. >> >> I would next run the code with valgrind to insure there are no issues of >> memory corruption or un-initialized data. >> >> How are you computing >> >> (dp/dt)*(Pxx+Pyy+Pzz) >> >> >> That is, how are you computing Pxx etc? >> >> Are you using finite elements for the U and P model? Exactly what elements? >> > > I agree with Barry. This does not seem to make sense, so I would expect > some kind of inconsistent discretization, or other > mathematical problem which makes your system unsolvable.
Try -mat_fd_type ds before ruling out sensitivity to differencing parameter.
