[petsc-users] TS question 1: how to stop explicit methods because they do not use SNES(VI)?

[Jed Brown]

Criminy, that's disgusting. You implement options

-term1 rhs -term2 lhs ...

In your FormIFunction:

if (term1 == LHS) { ... }

and similarly in your FormRHSFunction.

On February 14, 2017 1:55:42 PM MST, Emil Constantinescu <emconsta at mcs.anl.gov> wrote:
>On 2/14/17 2:33 PM, Zhang, Hong wrote:
>> I think many users (including me) would like to start with academic
>examples, e.g. u_t=f(u)+g(u), when they try to learn PETSc TS solvers.
>This simple form allows for easy switch between all kinds of different
>integration methods.
>
>Right, but then you can just write an if statement along the lines in 
>ex31.c: (this was before RHSJacobian and it's not intended for IMEX)
>
>TSGetType(ts,&time_scheme);
>if ((!strcmp(time_scheme,TSEULER)) || (!strcmp(time_scheme,TSRK)) || 
>(!strcmp(time_scheme,TSSSP))) {
>/* Explicit time-integration -> specify right-hand side function ydot =
>
>f(y) */
>TSSetRHSFunction(ts,NULL,RHSFunction,&ptype[0]);
>} else if ((!strcmp(time_scheme,TSTHETA)) ||...
>               (!strcmp(time_scheme,TSARKIMEX))) {
>      /* Implicit time-integration -> specify left-hand side function 
>ydot-f(y) = 0 */
>      /* and its Jacobian function                   */
>      TSSetIFunction(ts,NULL,IFunction,&ptype[0]);
>      TSSetIJacobian(ts,Jac,Jac,IJacobian,&ptype[0]);
>    }
>
>This should not be a performance bottleneck. IFunction can be a wrapper
>
>of the RHSFunction so not too much extra coding.
>
>Emil
>
>> Experienced users or experts who need to solve DAEs or complicate
>problems involving nontrivial mass matrix should be encouraged to try
>IFunction/IJacobian. Of course, we need better documentation to help
>them realize the switching is not always possible. Personally it took
>me a long time to get used to the IFunction business since I had been
>using u_t=f(u)+g(u) for a couple years before I jumped on PETSc. If we
>add support for this simple form, the learning curve would be less
>steep than it currently is.
>>
>> Hong (Mr.)
>>
>>> On Feb 14, 2017, at 11:55 AM, Jed Brown <jed at jedbrown.org> wrote:
>>>
>>> Barry Smith <bsmith at mcs.anl.gov> writes:
>>>>  Hence my suggestion to have TSSetLeftHandSideFunction() (or Jed's
>suggestion to have multiple Right Hand side functions) this will allow
>comparison of implicit, explicit, imex without recompiling (which we
>don't have currently) for the case with no mass matrix. With a mass
>matrix, unless we use mass lumping and have a TSSetMassMatrix() you
>cannot switch to full explicit, but that is due to mathematics, not the
>interface.
>>>
>>> Who in reality has a problem that looks like
>>>
>>>  u_t = f(u) + g(u) + h(u)
>>>
>>> where it may make sense to move some of this to the left (implicit)?
>>> I'm concerned that this isn't natural for many applications so
>adding an
>>> interface would be solving a problem that doesn't really exist
>outside
>>> perhaps a few academic examples.
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