[petsc-dev] Proposed changes to TS API

[Matthew Knepley]

On Fri, May 11, 2018 at 4:36 PM, Jed Brown <jed at jedbrown.org> wrote:

> "Zhang, Hong" <hongzhang at anl.gov> writes:
>
> > We are not forcing users to do two matrix assemblies per time
> > step. For most cases, there is even no need to update dF/dUdot at
> > all. For extreme cases that the application requires frequent update
> > on dF/dUdot and assembly is expensive, one can always assemble the
> > single-matrix Jacobian and throw it to SNES directly.
>
> For the vast majority of cases, the mass matrix terms are inexpensive
> and can be summed during each assembly at negligible cost (cheaper than
> accessing those terms from an already-assembled matrix).
>
> >
> > TS used to be an unusable pile of crap until Jed proposed the marvelous
> > IJacobian interface. Suddenly COMPLEX fully-implicit DAE problems become
> > SIMPLE to express, and a single IFunction/IJacobian pair reusable for
> > different timestepper implementations a reality. And we got an added
> > bounus: this was efficient, it involved a SINGLE global matrix assembly.
> If
> > the issue is in supporting simpler problems, then we should go for an
> > alternative interface with broken Jacobians, just as Stefano propossed
> in a
> > previous email. I'm totally in favor of an additional broken Jacobians
> API,
> > and totally againt the removal of the single-matrix IJacobian interface.
> >
> > The current IJacobian is essentially SNESJacobian. And the single-matrix
> SNESJacobian interface is always there. Power users could set up the
> SNESJacobian directly if we pass a read-only shift parameter to them. So we
> are by no means prohibiting power users from doing what they want.
>
> You mean the user would call TSGetSNES and SNESSetJacobian, then
> internally call TSGetIJacobianShift and some new function to create
> Udot?  That sounds way messier and error-prone.


And completely impossible to compose. We should be explicitly talking about
subsystems that use TS. For example,
I have a nonlinear system for plasticity. I want to use a preconditioner
that introduces some elasticity and timesteps to
steady state to provide a good Newton direction. I need to to be able to
create the solver without all sorts of digging
in and setting things. This idea that you "just set SNESJacobian" is a
non-starter.

  Matt


>
> > IJacobian with shift mixes TS Jacobian and SNES Jacobian. This is the
> issue we need to fix.
>
> It is just one interface producing exactly the matrix that the solver
> needs.
>



-- 
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/>
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