dolfin team mailing list archive

[Johan Hake <668862@xxxxxxxxxxxxxxxxxx>]

On Saturday November 20 2010 10:02:36 Garth Wells wrote:
> I've done some testing, and using unordered_set appears to not perform
> as well as Set for standard problems.

What are the differences in timing here? The difference in timing using 
unordered_set for Real spaces where dramatically better.

> Each dof (matrix row) has its own set. The sets are usually small, and
> independent of the mesh size. The problem is that the size of set that
> belongs to the global dof is equal to the number of columns in the
> matrix, and when looping over this set (which becomes very large)
> std::find is called a lot.

Is this true for any form? Why is this so notable when computing the sparsity 
pattern when a subspace belongs to Real?

> I don't see simple solution from the SparsityPattern side. What's
> probably needed is an extension to UFC to distinguish between local and
> global dofs.

How would this help?

Johan

-- 
Sparsity pattern computation dead slow with 'real' spaces
https://bugs.launchpad.net/bugs/668862
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Status in DOLFIN: Confirmed

Bug description:
The computation of the sparsity pattern is dead slow when real spaces are used, i.e.,

    R = FunctionSpace(mesh, 'R', 0)

To see the problem, compare the neumann-poisson and poisson demos with fine meshes (256x256)