dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

On Oct 18 2009, mspieg wrote:

> On Oct 16, 2009, at 6:04 PM, Anders Logg wrote:
>
> > On Fri, Oct 16, 2009 at 03:34:06PM -0400, mspieg wrote:
> > > Hi All,
> > >   I'm trying to update a working 0.9.3 code to  0.9.4, and I need  
> > > to understand
> > > the new interfaces for Function::eval a bit better.
> > >
> > > I had things working fine with the old interface
> > >
> > > Function::eval (double *values, const double *x, const ufc::cell  
> > > &ufc_cell,
> > > uint cell_index)
> > >
> > > and am trying to understand how to efficiently use
> > >
> > > Function::eval (double *values, const double *x, const Cell  
> > > &dolfin_cell, const
> > > ufc::cell &ufc_cell)
> > >
> > > (My codes need to do a large number of evals at arbitrary points  
> > > and cells
> > > efficiently)
> > >
> > > among other things, is there an easy way to update the index of a  
> > > cell, without
> > > constructing a new Cell every time? (or shouldn't I be worrying  
> > > about this).
> >
> > Constructing a cell should not cost that much. It just stores the
> > index, the dimension and a reference to the mesh.
> >
> > If you are using iterators over a mesh, then you already have the cell
> > by dereferencing the pointer. The typical usage would be
> >
> > UFCCell ufc_cell(mesh);
> > for (CellIterator cell(mesh); !mesh.end(); ++cell)
> > {
> >   ufc_cell.update(*cell);
> >
> >   f.eval(values, x, *cell, ufc_cell);
> > }
> >
> > But perhaps you don't use iterators?
>
> Thanks Anders,
>   I haven't been using iterators because I've been overloading eval 
> (double* values, Data& data) and getting the cells passed through  
> data during assembly
>
> the issue is that I'm not necessarily evaluating at the position and  
> cell in data, but at another point and cell derived from the current  
> location and cell (it's basically a particle tracking problem).
>
> Ideally, given data I would set another Data structure (data_prime)  
> with the new point, cell and ufc_cell, then call eval as needed.  As  
> long as the cost to construct a cell is small, it will probably be  
> easy enough to do with the current code (the problem is I need to do  
> this at every quadrature point, and I found previously that  
> construction of UFCCell at every point was exceptionally expensive).

Yes, a UFCCell is expensive to construct as it allocates memory 
dynamically. UFCCell::update should be cheap though.

 UFCCell ufc_cell(mesh);
 for (CellIterator . . . . )
 {
   ufc_cell.update(*cell);
 }

Garth

> Anyway, I'll play around a bit more and if I have specific issues or  
> design questions, I'll let you know.
>
> cheers
> marc
>
>
>
>
> > > Anyway, I need to play with things a bit more, but looking at the  
> > > source code
> > > for eval,  I was also wondering
> > >  if lines 320 and 321 are required as line 322 doesn't seem to  
> > > require cell.
> > >
> > >  typo? bug? or general ignorance on my part (probably the latter).
> >
> > Yes, lines 320-321 should be removed. Thanks for noticing. I've fixed
> > this now.
> >
> > --
> > Anders
> >
> >
> > > cheers
> > > marc
> > >
> > > void Function::eval(double* values, const Data& data) const
> > >
> > > 00311 {
> > > 00312   assert(values);
> > > 00313   assert(_function_space);
> > > 00314
> > > 00315   // Check if UFC cell if available and cell matches
> > > 00316   if (data._dolfin_cell && _function_space->has_cell 
> > > (*data._dolfin_cell))
> > > 00317   {
> > > 00318     // Efficient evaluation on given cell
> > > 00319     assert(data._ufc_cell);
> > > 00320     const uint cell_index = data._ufc_cell->entity_indices 
> > > [data._ufc_cell->topological_dimension][0];
> > > 00321     Cell cell(_function_space->mesh(), cell_index);
> > > 00322     eval(values, data.x, *data._dolfin_cell, *data._ufc_cell);
> > > 00323   }
> > > 00324   else
> > > 00325   {
> > > 00326     // Redirect to point-based evaluation
> > > 00327     eval(values, data.x);
> > > 00328   }
> > > 00329 }
> > >
> > > ----------------------------------------------------
> > > Marc Spiegelman
> > > Lamont-Doherty Earth Observatory
> > > Dept. of Applied Physics/Applied Math
> > > Columbia University
> > > http://www.ldeo.columbia.edu/~mspieg
> > > tel: 845 704 2323 (SkypeIn)
> > > ----------------------------------------------------
> >
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> >
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>
> ----------------------------------------------------
> Marc Spiegelman
> Lamont-Doherty Earth Observatory
> Dept. of Applied Physics/Applied Math
> Columbia University
> http://www.ldeo.columbia.edu/~mspieg
> tel: 845 704 2323 (SkypeIn)
> ----------------------------------------------------