-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I guess I haven't made my point very clear. The demo you gave is a good example of what's currently possible and where the limit is I think.
Say you take dolfin/demo/pde/spatial-coordinates/cpp and want to restrict the source term f to only a circle in the center of the domain. How would you do that? I wouldn't know another way than a conditional and I couldn't find a way to get that working with FFC. I was not talking about constants, but about expressions that are not uniform over the whole domain, should have made that more clear. Florian On 23.07.2010 13:51, Kristian Ølgaard wrote: > On 23 July 2010 12:40, Florian Rathgeber > <[email protected]> wrote: > Hi, > > Profiling DOLFIN assembly shows that for forms with many coefficients or > computationally expensive expressions assigned to coefficients, a great > deal of the whole assembly is spent in expression evaluation. > > An idea I wanted to investigate was giving an expresssion in UFL and > have FFC generate corresponding code to evaluate it inline during tensor > tabulation. That should have the potential of eliminating some of the > overhead. > > However, currently that is only possible within very limited bounds. FFC > does not yet support UFL conditionals, which essentially limits possible > expressions to depend on coordinates only and be the same over the whole > domain (which is not very useful for most form coefficients, e.g. source > terms). > >> Did you have a look at the dolfin/demo/pde/spatial-coordinates/cpp >> demo? It is essentially the dolfin/demo/pde/poisson/cpp demo but it >> uses source and boundary terms defined using coordinates and is not >> the same over the whole domain (I don't really get your point here). >> The coordinates are defined as the coordinates of the current >> integration point located on the current cell which is being >> integrated. Have a look at the generated code, then it might be >> clearer what's going on. > >> Kristian > > This brings me to my questions: > 1) Do you see a reasonable chance for speeding up assembly in the way > described? > 2) Is my assessement correct? Or do I miss some capabilities of FFC that > support my use case? > 3) Since UFL basically support arbitrary python syntax, would it be > reasonable to support translation for user-defined Expressions given in > python (either overloading eval in a class defived from Expression or > passing C++ code to the SWIG constructor of Expression)? FFC would > generate code to evaluate these Expressions inline during tensor > tabulation. That would of course require the Expression to be stateless > and have not input other than the coordinates. But at least it would > provide much more flexibility and also consistency with the DOLFIN > interface in general > (sidenote: it is possible to specify Expressions in this way in UFL, but > they are treated just like Coefficients by FFC) > > Illustration example (UFL file): > from dolfin import Expression > > code_expr = """ > class Source : public Expression > { > public: > void eval(Array<double>& values, const Array<double>& x) const > { > values[0] = ((x[0]-0.25)*(x[0]-0.25)+(x[1]-0.5)*(x[1]-0.5) < > 0.025) ? 1.0 : 0.0; > } > }; > """ > > scalar = FiniteElement("Lagrange", triangle, 1) > > v = TestFunction(scalar) > u = TrialFunction(scalar) > u0 = Coefficient(scalar) > f = Expressions(code_expr) > > c = 0.05 > k = 0.1 > alpha = 1.0 > > a = v*u*dx + k*alpha*u*v*dx + k*c*dot(grad(v), grad(u))*dx > L = v*u0*dx + k*v*f*dx > > Florian >> >> _______________________________________________ Mailing list: https://launchpad.net/~ffc Post to : [email protected] Unsubscribe : https://launchpad.net/~ffc More help : https://help.launchpad.net/ListHelp >> >> -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.12 (MingW32) iEYEARECAAYFAkxJhNoACgkQ8Z6llsctAxYaOgCeNN1gpBh2h3QzTrnevHMcKcXn gXAAn0SaoqOou75hAayf8+Y8Y5cVZurn =tNNo -----END PGP SIGNATURE-----
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