[petsc-users] Question about TSComputeRHSJacobianConstant

[Zhang, Hong]

Hi Sajid,

Can you please try this branch hongzh/fix-computejacobian quickly and see if it makes a difference?

Thanks,
Hong (Mr.)

On May 16, 2019, at 8:04 PM, Sajid Ali via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> 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).

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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