[petsc-dev] TS Terminology

[Emil Constantinescu]

On 10/20/17 9:11 AM, Matthew Knepley wrote:
> On Fri, Oct 20, 2017 at 9:23 AM, Emil Constantinescu 
> <emconsta at mcs.anl.gov <mailto:emconsta at mcs.anl.gov>> wrote:
> 
>     On 10/20/17 7:57 AM, Matthew Knepley wrote:
> 
>         I am confused by some of the terminology in TS. At the top
>         level, IFunction appears to mean the entire equation
> 
>             F(u, u_t, x) = 0
> 
> 
>     Matt, page 141 of the manual: F(t, u, u_t) = G(t, u), and not zero
>     on the RHS side. To make the interface general we allow internally
>     for F:= F(t, u, u_t) - G(t, u) and then F=0.
> 
> 
> This is not "internal". Its the toplevel interface:
> 
> https://bitbucket.org/petsc/petsc/src/63ae3ecac3af8ce782273a76ad4152cddc2fd80a/src/ts/interface/ts.c?at=master&fileviewer=file-view-default#ts.c-884
> 
> It computes F - G.

That's what it should do in some cases. The user provides either 
ifunction or rhs funtion or both. The api to the solvers can take care 
of this stuff automatically - that's what I meant by internal. Different 
TS solvers can take different definitions of the funtions; e.g., imex 
need both, beuler can take ifuntion and/or rhs function but instead of 
writing beuler for both we choose the most general case (ifunction) and 
compose the functions accordingly. The F - G is transparent to the user. 
But somewhere the sausage needs to be made and I think that is the right 
level because that is least likely to change and least maintenance.

Emil

>     Matt
> 
>     Emil
> 
>     PS: Manual: PETSc 3.8 September 26, 2017
> 
>         However, this appears to mean something different than ifunction
>         at the function pointer level. Inside TSCompiteIFunction(), it
>         uses both ifunction and rhsfunction. This makes it hard to
>         understand how composition works. TSComputeRHS() is called
>         inside TSComputeIFunction(), so if we want to reuse vectors it
>         should not initialize the vector, but it seems like
>         TSComputeIFunction() should initialize the vector since
>         TSComputeIFunctionLocal() does.
> 
>         What guarantees about initialization should we have?
> 
>              Matt
> 
>         -- 
>         What most experimenters take for granted before they begin their
>         experiments is infinitely more interesting than any results to
>         which their experiments lead.
>         -- Norbert Wiener
> 
>         https://www.cse.buffalo.edu/~knepley/
>         <https://www.cse.buffalo.edu/~knepley/>
>         <http://www.caam.rice.edu/~mk51/ <http://www.caam.rice.edu/~mk51/>>
> 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their 
> experiments is infinitely more interesting than any results to which 
> their experiments lead.
> -- Norbert Wiener
> 
> https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>