dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/10/22 Johan Hake <hake@xxxxxxxxx>:
> On Wednesday 22 October 2008 10:17:43 Martin Sandve Alnæs wrote:
>> 2008/10/22 Johan Hake <hake@xxxxxxxxx>:
>> > On Wednesday 22 October 2008 09:32:31 Martin Sandve Alnæs wrote:
>> >> 2008/10/22 Johan Hake <hake@xxxxxxxxx>:
>> >> > On Tuesday 21 October 2008 23:23:27 Martin Sandve Alnæs wrote:
>> >> >> 2008/10/21 Johan Hake <hake@xxxxxxxxx>:
>> >> >> > On Tuesday 21 October 2008 22:34:04 Martin Sandve Alnæs wrote:
>> >> >> >> 2008/10/21 Johan Hake <hake@xxxxxxxxx>:
>> >> >> >> > On Tuesday 21 October 2008 21:37:13 Martin Sandve Alnæs wrote:
>> >> >> >> >> 2008/10/21 Anders Logg <logg@xxxxxxxxx>:
>> >> >> >> >> > On Tue, Oct 21, 2008 at 06:01:53PM +0100, Garth N. Wells
> wrote:
>> >> >> >> >> >> Anders Logg wrote:
>> >> >> >> >> >> > On Tue, Oct 21, 2008 at 04:45:01PM +0100, Garth N. Wells
>> >
>> > wrote:
>> >> >> >> >> >> >> I have a few questions and thoughts regarding the new
>> >> >> >> >> >> >> Function design
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> * It's not clear to me what the intention is with
>> >> >> >> >> >> >> user-defined functions. The functions
>> >> >> >> >> >> >> Function::interpolate(...) never call eval(..), so they
>> >> >> >> >> >> >> can't pick up user-defined values. Should
>> >> >> >> >> >> >> Function::interpolate test for the presence of a
>> >> >> >> >> >> >> GenericVector to decide whether or not the Function is
>> >> >> >> >> >> >> discrete or user-defined?
>> >> >> >> >> >> >
>> >> >> >> >> >> > Yes, sorry. I've missed this. I'll fix it.
>> >> >> >> >> >> >
>> >> >> >> >> >> >> * It would be useful to declare user-defined functions
>> >> >> >> >> >> >> without associating a FunctionSpace. If we want to
>> >> >> >> >> >> >> interpolate the function, a FunctionSpace must then be
>> >> >> >> >> >> >> provided. Anyone see any problems with this?
>> >> >> >> >> >> >
>> >> >> >> >> >> > The reasoning here is that all Functions must always be
>> >> >> >> >> >> > associated with a FunctionSpace so that they may be
>> >> >> >> >> >> > correctly interpreted in forms and correctly plotted. When
>> >> >> >> >> >> > a Function is created in PyDOLFIN, it must always be
>> >> >> >> >> >> > associated with a certain FiniteElement (and in a while
>> >> >> >> >> >> > FunctionSpace). It would simplify the handling of
>> >> >> >> >> >> > Functions if they are always associated with a
>> >> >> >> >> >> > FunctionSpace.
>> >> >> >> >> >>
>> >> >> >> >> >> I agree that is makes life simple if every function has a
>> >> >> >> >> >> space, but it is a bit clunky for declaring user-defined
>> >> >> >> >> >> functions. The forms must be declared first to extract the
>> >> >> >> >> >> finite element to create the function space. Could look
>> >> >> >> >> >> nasty when a lot of functions are involved.
>> >> >> >> >> >>
>> >> >> >> >> >> We have a function Function::interpolate which takes a
>> >> >> >> >> >> function space V as an argument and it interpolates the
>> >> >> >> >> >> function u in V. What if we permit undefined function spaces
>> >> >> >> >> >> (which perhaps only have a domain)? We would then
>> >> >> >> >> >> interpolate the user defined function u in the provided
>> >> >> >> >> >> space V.
>> >> >> >> >> >>
>> >> >> >> >> >> Garth
>> >> >> >> >> >
>> >> >> >> >> > Are user-defined functions ever used without being related to
>> >> >> >> >> > a particular element/function space?
>> >> >> >> >> >
>> >> >> >> >> > It don't think it will be very clumsy. The clumsy thing will
>> >> >> >> >> > be to (in C++) get from something compiled by a form compiler
>> >> >> >> >> > to a FunctionSpace.
>> >> >> >> >> >
>> >> >> >> >> > If we can make that operation smooth, then creating
>> >> >> >> >> > (user-defined) functions will be very simple and convenient.
>> >> >> >> >> > One just needs to supply the variable V holding the function
>> >> >> >> >> > space.
>> >> >> >> >> >
>> >> >> >> >> > The current way of extracting function space data from the
>> >> >> >> >> > form is not very nice (in C++). What would be the optimal way
>> >> >> >> >> > to initialize a FunctionSpace in C++? We could think of
>> >> >> >> >> > extending the code generation to generate code that makes
>> >> >> >> >> > this convenient.
>> >> >> >> >> >
>> >> >> >> >> > --
>> >> >> >> >> > Anders
>> >> >> >> >>
>> >> >> >> >> The current way of extracting function space data from the form
>> >> >> >> >> is not very nice in Python either, since it doesn't work with
>> >> >> >> >> compiled functions. (Never mind that the current code is
>> >> >> >> >> FFC-specific, this will be the same with UFL).
>> >> >> >> >>
>> >> >> >> >> Using Python functors can easily make the assembly slower than
>> >> >> >> >> solving the linear system, so it's not really interesting to do
>> >> >> >> >> in real applications...
>> >> >> >> >>
>> >> >> >> >> To make a function object that is both of a C++ subclass of
>> >> >> >> >> dolfin::Function and of the Python class ufl.Function, we can't
>> >> >> >> >> use the fixed multiple inheritance
>> >> >> >> >> solution in the current PyDOLFIN.
>> >> >> >> >>
>> >> >> >> >> We would have to define a new class dynamically in python,
>> >> >> >> >> inheriting from both ufl.Function and the freshly compiled C++
>> >> >> >> >> Function subclass. After all this work cleaning up the Function
>> >> >> >> >> class hierarchy, is that really something you want?
>> >> >> >> >>
>> >> >> >> >> I'm not sure if that is even possible to do while maintaining
>> >> >> >> >> efficiency, with cross-language inheritance and SWIG directors
>> >> >> >> >> and all that.
>> >> >> >> >>
>> >> >> >> >> If anyone has another solution, I'm very interested in hearing
>> >> >> >> >> it! Otherwise, I'm all for keeping the ufl.Function objects
>> >> >> >> >> used in form definition separated from dolfin.Function objects
>> >> >> >> >> used in assembly.
>> >> >> >> >
>> >> >> >> > I agree with Martin that we need to have a solution for PyDOLFIN
>> >> >> >> > users that does not depend on using python functors, as it will
>> >> >> >> > take forever for a complex form together with a moderate mesh to
>> >> >> >> > just assemble the form.
>> >> >> >> >
>> >> >> >> > Is it possible to let compile_functions compile a cpp function,
>> >> >> >> > with a FunctionSpace and all, instead of a mesh as it is today.
>> >> >> >> > Then after doing
>> >> >> >>
>> >> >> >> If you have a dolfin::FunctionSpace object already, there's no
>> >> >> >> reason compile_functions can't take this instead of dolfin::Mesh.
>> >> >> >> That's exactly the same and no problem at all.
>> >> >> >>
>> >> >> >> > this compile_function extract the element, and instantiate a
>> >> >> >> > UFL/FFC/PyFunction-function, and "attach" the compiled version
>> >> >> >> > to it. This
>> >> >> >>
>> >> >> >> What I state above is that this "attachment" must be done with
>> >> >> >> dynamic creation of a new class with multiple inheritance.
>> >> >> >> And I am unsure whether this will work out properly with SWIG
>> >> >> >> directors etc. I believe it _may_ work, but I don't dare to keep
>> >> >> >> my hopes up :-)
>> >> >> >
>> >> >> > Ok, I get it. For a moment I thought we could get away by defineing
>> >> >> > our own PyDOLFIN::Function class that could inherit from UFL/FFC,
>> >> >> > and then have a cpp_Function, but I realise this will not work.
>> >> >> >
>> >> >> >> See the attached python file for a prototype of dynamic class
>> >> >> >> creation with multiple inheritance using pure python classes.
>> >> >> >> (I think this is called "aspect oriented programming" by some
>> >> >> >> people)
>> >> >> >>
>> >> >> >> > can be used to define forms, but more important it can be handed
>> >> >> >> > to the python assembly that check if the function has a compiled
>> >> >> >> > version attached to it and send this to the cpp_assembler?
>> >> >> >>
>> >> >> >> If the "attachment" is anything other than inheritance, it will
>> >> >> >> have to be checked with manually written python code _everywhere_
>> >> >> >> a dolfin::Function is expected... We can't have one kind of
>> >> >> >> functions for assembly and one for other stuff.
>> >> >> >
>> >> >> > Ok, I guess we have three different cases:
>> >> >> >
>> >> >> >  1) PyFunctions inherting from both UFL/FFC and cpp_Function as
>> >> >> > today, taking a functionsspace in its constructor. This will work
>> >> >> > with both user defined and discrete functions, more or less as we
>> >> >> > have it today.
>> >> >> >
>> >> >> >  2) The special functions, MeshSize, etc, can also be defined in
>> >> >> > the same way as now, right?
>> >> >> >
>> >> >> >  3) Using compile_functions, that creates a multi inheritance
>> >> >> > object that can be sent to any function expecting a cpp_Function,
>> >> >> > without manually extending the python interface.
>> >> >>
>> >> >> I'm with you up to this point.
>> >> >>
>> >> >> > Could the last be done by letting compile_function create a muliti
>> >> >> > inheritance Function. Instantiate the cpp_one with the function
>> >> >> > space and by that creating a dummy cpp_function. Then "attach" the
>> >> >> > compiled function to a protected attribute and define eval, by
>> >> >> > overloading it in python. This will then just call the attached and
>> >> >> > compiled cpp_functions eval.
>> >> >>
>> >> >> What you describe here sounds like the envelope-letter design
>> >> >> that was just _removed_ from dolfin.
>> >> >
>> >> > Yes, but only for compiled functions in Python. No other places.
>> >> >
>> >> >> What I'm suggesting is that
>> >> >> compile_functions dynamically creates a Python class that inherits
>> >> >> from ufl.Function and the freshly compiled C++ class, which is
>> >> >> a dolfin::Function subclass. Then it can construct an object of this
>> >> >> new class, passing a FunctionSpace object given by the user to
>> >> >> the dolfin::Function constructor, and an ufl.FiniteElement to the
>> >> >> ufl.Function constructor.
>> >> >
>> >> > This sounds doable. I realize now that this was what you were talking
>> >> > about in your previous emails, but I did not get it until now ;)
>> >> >
>> >> >> This of course requires that dolfin.FunctionSpace
>> >> >> is a Python subclass of dolfin::FunctionSpace with an additional
>> >> >> ufl.FiniteElement member variable. Using jit, dolfin.FunctionSpace
>> >> >> can compile the ufc::finite_element and ufc::dof_map classes it needs
>> >> >> from an ufl.FiniteElement. And then there's the issue of reusing
>> >> >> dofmaps, where DofMapSet enters the play...
>> >> >
>> >> > Do we need to jit compile ufc::finite_elements and ufc::dof_maps from
>> >> > the created ufl.FiniteElement? What about the one that follows from
>> >> > the FunctionSpace?
>> >>
>> >> I was thinking about when _constructing_ the FunctionSpace.
>> >> Just like PyDOLFIN uses jit in Function.__init__ today.
>> >
>> > Ok, something like:
>> >
>> > # Note pseudo code...
>> > class FunctionSpace(cpp_FunctionSpace):
>> >    def __init__(self,ufl_finite_element,mesh):
>> >        ufc_finit_element = jit(ufl_finite_element)
>> >        form = ufl.FiniteElement*ufl.TestFunction*ufl.dx
>> >        dof_map = jit(form)
>> >        cpp_FucntionSpace.__init__(mesh,ufc_FinitElement,dof_map)
>> >        self._UFL_FiniteElement = ufl_finite_element
>> >
>> >    def UFL_FiniteElement(self):
>> >        return self._UFL_FiniteElement
>> >
>> > By this the the ufc_element, ufl_element, the dofmaps and the mesh, are
>> > cached in the FunctionSpace.
>> >
>> > The Function would then be something like:
>> >
>> > class Function(cpp_Function,ufl.Function):
>> >    def __init__(self,function_space):
>> >        cpp_Function.__init__(function_space):
>> >        ufl.Function.__init__(function_space.UFL_FiniteElement())
>> >
>> > and dynamical created code in compile_functions()
>> >
>> > class MyFunction(MyCompiledFunction,ufl.Function):
>> >    def __init__(self,function_space):
>> >        MyCompiledFunction.__init__(function_space):
>> >        ufl.Function.__init__(function_space.UFL_FiniteElement())
>>
>> Something like that, yes. This is close to the current PyDOLFIN.
>>
>> But FunctionSpace might become a subclass of ufl.FunctionSpace
>> if we introduce that in UFL, and it should be possible to get
>> cached initialized and renumbered DofMaps from a DofMapSet.
>>
>> Since a DofMapSet will typically be initialized with a Form,
>> a Form depends on a Function, and a Function depends on
>> a FunctionSpace which should be initialized by the DofMapSet,
>> we have a cirular dependency right there.
>
> But won't you have this circular dependency in UFL already?

In UFL this is simple:

  FiniteElement depends on nothing
  Function depends on FiniteElement
  Form depends on Function

The relation between UFL and UFC code is (at a certain level) simple:

  ufc::* is generated by form compilers from ufl.* (equivalently ffc.*)
  once generated, ufc::* depends on nothing

In DOLFIN (C++) it is also simple:

  dolfin::* does not depend directly on ufl.* (equivalently ffc.*)
  dolfin::* depends on ufc::*

PyDOLFIN could (should!) be kept simple. I've been using PyDOLFIN
without the FFC-dependent multiple inheritance stuff all the time
with the dolfin.cpp_* classes. I believe that's also how it must be
done if you have external UFC code, e.g. precompiled in a library
or manually written.

So no matter what, it should at least be possible to avoid
relying on the multiple inheritance and JIT in PyDOLFIN.

--
Martin