On 13/06/11 12:51, Marie E. Rognes wrote: > On 06/13/2011 10:03 AM, Garth N. Wells wrote: >> >> >> On 12/06/11 22:54, Anders Logg wrote: >>> On Wed, Jun 08, 2011 at 11:47:10PM +0200, Anders Logg wrote: >>>> I'm on a cell phone and can't engage in any discussions but I just >>>> want to throw something in. Marie and I discussed at some point >>>> another option which is to write everything as F = 0 and let UFL >>>> compute J even for linear problems. J would be a optional >>>> variable. Then we could have a common interface and also a common >>>> solver. >>> >> >> I think that this would be a nice option, but I don't think that it can >> work without a more elaborate interface than just >> >> pde = VariationalProblem(F, bcs) >> >> because UFL cannot know which coefficient in a form is unknown, and >> therefore which function to compute the linearisation with respect to. >> Moreover, >> >> - UFL cannot compute the linearisation for all forms >> - In some cases it's not desirable to let UFL do the linearisastion >> > > Some specific responses: First, the moment solve is called (with a > coefficient), which function to compute the linearization with respect > to, is known.
Which means that the simple call u = pde.solve() will not be possible. > Second, the Jacobian could definitely be provided as an > optional argument for those cases where it is not feasible/desirable > for UFL to do the linearization. See below for longer, more general > rant. > >>> Any further thoughts on this? It's important we get this right. >>> >>> I can live with >>> >>> LinearVariationalProblem >>> NonlinearVariationalProblem >>> >>> if everyone else thinks that's the best option, > > I don't think it is the best option. > >>> but don't really like >>> it since it's very long. > > My reasons do not really relate to the length of the name, but rather > that I do not see why we should make the difference between linear and > nonlinear variational problems greater in the _highest level > interface_ of DOLFIN. I think this just makes things more cumbersome > (a) in terms of debugging and (b) in terms of explaining how to solve > variational problems to FEniCS beginners. > My experience is the complete opposite of this. > My typical pedagogical use case looks something like this (in Python): > Say you have explained to someone how to solve stationary Stokes with > Taylor-Hood, and then want to extend to Navier-Stokes. Then ideally > all that should be necessary is to add the nonlinear term to the > form(s). But no. In addition, you at least have to replace the > TrialFunction with a Function (which typically leads to a longer > discussion on what a TrialFunction represents and why that is used for > linear problems when the same ansatz is made for the two in most text > books), and then you have to call solve with the same Function instead > of just returning it (which is also on my top 3 list of pitfalls). I > don't see how adding > LinearVariationalProblem/NonlinearVariationalProblem to the list makes > it better. The same (but the other way) goes when wanting to debug > Navier-Stokes by reducing to Stokes. > > In essence, I've started treating all variational problems as > nonlinear. > This might be fine for toy problems, but not for real problems. There is overhead in a nonlinear solve to check for convergence, which is not required if the eqn is linear. Garth > -- > Marie > >>> Another option (if we don't go for the F = 0 >>> option) is >>> >>> VariationalProblem >>> NonlinearVariationalProblem >>> >> >> I prefer to the long one. I don't see any advantage in favouring a short >> name over a more explicit and readable name. >> >> Garth >> >>> -- >>> Anders >>> >>> _______________________________________________ >>> Mailing list: https://launchpad.net/~dolfin >>> Post to : dolfin@lists.launchpad.net >>> Unsubscribe : https://launchpad.net/~dolfin >>> More help : https://help.launchpad.net/ListHelp >> >> >> _______________________________________________ >> Mailing list: https://launchpad.net/~dolfin >> Post to : dolfin@lists.launchpad.net >> Unsubscribe : https://launchpad.net/~dolfin >> More help : https://help.launchpad.net/ListHelp > > > _______________________________________________ > Mailing list: https://launchpad.net/~dolfin > Post to : dolfin@lists.launchpad.net > Unsubscribe : https://launchpad.net/~dolfin > More help : https://help.launchpad.net/ListHelp _______________________________________________ Mailing list: https://launchpad.net/~dolfin Post to : dolfin@lists.launchpad.net Unsubscribe : https://launchpad.net/~dolfin More help : https://help.launchpad.net/ListHelp
