dolfin team mailing list archive

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

Garth N. Wells wrote:
> Anders Logg wrote:
> > On Fri, Apr 27, 2007 at 05:24:34PM +0200, Garth N. Wells wrote:
> > > Anders Logg wrote:
> > > > I'm comparing the entries in the Stokes systems between the new code
> > > > and the old and there seem to be quite a few more zeros in the sparse
> > > > matrix than before.
> > > Strange. Have you counted the non-zeroes, or are you displaying the 
> > > matrix?
> > Displaying the matrix and counting everything that looks like zero.
> > (Just looking at the numbers.)
> There are about double the number of non-zeros (you can get the number 
> by A.nnz()) for the Stokes example with the new version compared to 
> version 0.6.4. For poisson and elasticity, the number of non-zeroes is 
> the same as with version 0.6.4. The linear algebra looks fine - 
> something is going on with the computation of the sparsity pattern for 
> the Stokes problem.
This is due to the significant number of zeroes in the element matrix 
for the Stokes problem. Previously, for uBLAS matrices the stiffness 
matrix was assembled into a temporary matrix A_temp which was then 
copied to a compressed row matrix A
  A.assign(A_temp);

The assign operation removed all non-zero terms. We no longer assemble 
into the temporary matrix.
Perhaps FFC could produce something to indicate the non-zero terms in 
the element matrix, although this would make things more complicated.
Garth

> Garth
>
>
> > /Anders
> >
> >
> > > Garth
> > >
> > > > Could this have anything to do with the initialization of uBlas
> > > > matrices with a sparsity pattern and not using the temporary storage
> > > > as before? Did we have a check that removed zeros?
> > > >
> > > > I haven't tried with PETSc.
> > > >
> > > > /Anders
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