[petsc-users] Question about TSComputeRHSJacobianConstant

[Sajid Ali]

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).

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