ffc team mailing list archive

["Garth N. Wells" <g.n.wells@xxxxxxxxxx>]

On Mon, 2006-05-29 at 19:32 +0200, Anders Logg wrote:
> On Mon, May 29, 2006 at 07:10:23PM +0200, Garth N. Wells wrote:
> > For evaluating functions at vertxes, FFC writes a function
> > 
> > void vertexeval(real values[], unsigned int vertex, const real x[],
> > const Mesh& mesh) const
> > 
> > Would there be a problem in changing "const real x[]" to "const
> > GenericVector x"? This would make it easier to represent a function
> > using different vector data structures.
> > 
> > Garth
> 
> The problem would be that vertexeval() would then need to call
> GenericMatrix::operator() (uint i) which is slow for PETScSparseMatrix.
> 
> But perhaps calling PETScSparseMatrix::array() is just as slow?
> 

When manipulating a lot of term, PETScSparseMatrix::array() is much
faster. I think that this has to with the calls

  VecAssemblyBegin(x);
  VecAssemblyEnd(x);

after accessing each entry.

> vertexeval() is just a temporary fix for Lagrange elements. Somehow,
> we need to compute the values at all vertices so we can write the
> solution to file. What we should really do would be to compute the
> projection onto piecewise linears and then just pick the values.
> 

OK. A simple temporary fix would be if vextexeval returned an array of
integers containing the positions in the big vector. Then the values
could be pulled as a block out of the vector. 

> Maybe we could always generate code for the projection onto piecewise
> linears when a form is compiled in FFC? If U is in V' and V is the
> space of piecewise linears, then we need to compute the projection
> W of U onto V:
> 
>     int v W dx = int v U dx for all v in V
> 
> On matrix form, we have
> 
>     MW = b
> 
> where b represents the right-hand side int U v dx.
> 
> The mass matrix M is already computed by default in DOLFIN so we don't
> need to generate it. We only need to generate the linear form v*U*dx.
> 

. . . and solve the linear system.

Garth

> Then we could remove vertexeval.
> 
> /Anders