Re: [petsc-users] ts behavior question

[Smith, Barry F. via petsc-users]

> On Nov 11, 2019, at 7:00 PM, Gideon Simpson via petsc-users 
> <petsc-users@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

> 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