[petsc-users] ts behavior question

[Zhang, Hong]

> On Nov 11, 2019, at 11:33 PM, Smith, Barry F. via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> 
> 
>> On Nov 11, 2019, at 7:00 PM, Gideon Simpson via petsc-users <petsc-users at mcs.anl.gov> wrote:
>> 
>> I noticed that when I am solving a problem with the ts and I am *not* using a da, if I want to use an implicit time stepping routine:
>> 1. I have to explicitly provide the Jacobian
> 
>   Yes

Alternatively, -snes_fd can be used to approximate the Jacobian with normal finite differences (no coloring). The FD approximation is not efficient, but should work for small problems, and it is also useful for testing your hand-written Jacobian (via -snes_test_jacobian) 

Hong (Mr.)

> 
>> 2. When I do provide the Jacobian, if I want to access the elements of x(t) to construct f(t,x), I need to use a const PetscScalar and a VecGetArrayRead to get it to work.
> 
>  Presumably you call VecGetArray() instead? 
>> 
>> 
>> 3.  My code works without declaring const when I'm using an explicit scheme.
>> 
>> In contrast, if I solve a problem using a da, my code works, I can use implicit schemes without having to provide the Jacobian, and I don't have to use const anywhere.
> 
>  The use with DMDA provides automatic routines for computing the needed Jacobians using finite differencing of your provided function and coloring of the Jacobian. This results in reasonably efficient computation of Jacobians that work in most  (almost all) cases.
>> 
>> Can someone clarify what is expected/preferred?
> 
>  You should always use VecGetArrayRead() for vectors you are accessing but NOT changing the values in. There is no reason not and it provides the potential for higher performance.
> 
>  The algebraic solvers have additional checks to prevent peopled from inadvertently changing the entries in x (which would produce bugs). Presumably this results in generating an error when you call VecGetArray(). At least some of the TS explicit calls do not have such checks. They could be added and should be added.  https://gitlab.com/petsc/petsc/issues/493
> 
>  Thanks for pointing out the inconsistency
> 
>  Barry
> 
>> 
>> -- 
>> gideon
>