> On May 11, 2018, at 3:38 PM, Matthew Knepley <knep...@gmail.com> wrote: > > On Fri, May 11, 2018 at 4:36 PM, Jed Brown <j...@jedbrown.org> wrote: > "Zhang, Hong" <hongzh...@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.
But this is exactly what TSComputeIJacobian currently does, so is the current interface 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/
