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

[Emil Constantinescu]

On 2/14/17 3:04 PM, Barry Smith wrote:

On Feb 14, 2017, at 2:55 PM, Emil Constantinescu <emcon...@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]);
  }

  Multiple yucks! Switching methods should not require compiling code except 
when absolutely necessary and this is not a case of absolutely necessary!

Wait, no. You don't recompile. When you set the -ts_type then it defines 
RHSFunction (for Explicit and Implicit) or RHS Function & IFunction (for 
IMEX). Adding all that logic to keep track of left sides and right sides 
for academic examples is likely not the best development.

Emil



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 <j...@jedbrown.org> wrote:

Barry Smith <bsm...@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.