[petsc-users] Strange behavior of TS after setting hand-coded Jacobian

[Barry Smith]

  It is definitely unexpected that -snes_test_jacobian verifies the Jacobian as matching but the solve process is completely different. 

   Please run with -snes_monitor -snes_converged_reason -ksp_monitor_true_residual -ksp_converged_reason -snes_test_jacobian and send all the output 

  Barry


> On Oct 25, 2021, at 9:53 AM, 仓宇 <yhcy1993 at gmail.com> wrote:
> 
> I'm using TS to solve a set of DAE, which originates from a
> one-dimensional problem. The grid points are uniformly distributed.
> For simplicity, the DMDA is not employed for discretization.
> 
> At first, only the residual function is prescribed through
> 'TSSetIFunction', and PETSC produces converged results. However, after
> providing hand-coded Jacobian through 'TSSetIJacobian', the internal
> SNES object fails (residual norm does not change), and TS reports
> 'DIVERGED_STEP_REJECTED'.
> 
> I have tried to add the option '-snes_test_jacobian' to see if the
> hand-coded jacobian is somewhere wrong, but it shows '||J -
> Jfd||_F/||J||_F = 1.07488e-10, ||J - Jfd||_F = 2.14458e-07',
> indicating that the hand-coded jacobian is correct.
> 
> Then, I added a monitor for the internal SNES object through
> 'SNESMonitorSet', in which the solution vector will be displayed at
> each iteration. It is interesting to find that, if the jacobian is not
> provided, meaning finite-difference is utilized for jacobian
> evaluation internally, the solution vector converges to steady
> solution and the SNES residual norm is reduced continuously. However,
> it turns out that, as long as the jacobian is provided, the solution
> vector will NEVER get changed! So the solution procedure stucked!
> 
> This is quite strange!  Hope to get some advice.
> PETSC version=3.14.6, program run in serial mode.
> 
> Regards
> 
> Yu Cang