[petsc-users] Question about TSComputeRHSJacobianConstant

[Smith, Barry F.]

> On May 16, 2019, at 8:04 PM, Sajid Ali <sajidsyed2021 at u.northwestern.edu> wrote:
> 
> While there is a ~3.5X speedup, deleting the aforementioned 20 lines also leads the new version of petsc to give the wrong solution (off by orders of magnitude for the same program). 

   Ok, sorry about this. Unfortunately this stuff has been giving us headaches for years and we are struggling to get it right.

> 
> I tried switching over the the IFunction/IJacobian interface as per the manual (page 146) which the following lines :

  It is probably better to not switch to the IFunction/IJacobian, we are more likely to get the TS version working properly.
> ```
> TSSetProblemType(ts,TSLINEAR);
> TSSetRHSFunction(ts,NULL,TSComputeRHSFunctionLinear,NULL);
> TSSetRHSJacobian(ts,A,A,TSComputeRHSJacobianConstant,NULL);
> ```
> are equivalent to :
> ```
> TSSetProblemType(ts,TSLINEAR);
> TSSetIFunction(ts,NULL,TSComputeIFunctionLinear,NULL);
> TSSetIJacobian(ts,A,A,TSComputeIJacobianConstant,NULL);
> ```
> But the example at src/ts/examples/tutorials/ex3.c employs a strategy of setting a shift flag to prevent re-computation for time-independent problems. Moreover, the docs say "using this function (TSComputeIFunctionLinear) is NOT equivalent to using TSComputeRHSFunctionLinear()" and now I'm even more confused.
> 
> PS : Doing the simple switch is as slow as the original code and the answer is wrong as well. 
> 
> Thank You,
> Sajid Ali
> Applied Physics
> Northwestern University