dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jan 15, 2007 at 10:54:18PM +0100, Garth N. Wells wrote:
> The extra time could be in the mesh initialisation. DofMap is pretty fast.

I'm not sure. The benchmark initializes the mesh connectivity before
assembling and then assembles 100 times, so at least the last 99 times
the mesh initialization should not do anything. (Calling init() on the
mesh twice does not do anything the second time.)

> What needs to be done is to use the DofMap to initialise the sparse 
> matrix pattern (at least for uBLAS). What happens now is that everything 
> is assembled into a uBLAS vector-of-compressed-vectors (fast assembly), 
> which is then copied to a CSR matrix (for fast matrix vector mult or for 
> passing to UMFPACK). Using DofMap, the CSR matrix could be initialised. 
> This should make things faster and use less memory

ok, let's do that.

> Also, which problem are you assembling? Scalar Poisson equation can be 
> misleading as it tends to fill sparse matrices quite fast even if they 
> are not well initialised. This tends to show up with vector equations.

Advection operator for cubic Lagrange in 3D:

    a = v*c[i]*U.dx(i)*dx

/Anders


> Garth
> 
> Anders Logg wrote:
> > There is a benchmark for assembly in src/bench/fem/assembly/.
> > We should use this to track the speed of assembly during the
> > planned changes to the Assembly and DofMap classes.
> > 
> > The assembly is now 59% slower than it was before the DofMap (and
> > maybe other changes).
> > 
> >   0.63-dev (changeset 1d485299d95)
> >   CPU time: 87.8056
> > 
> >   0.6.4 (changeset 49bf8f3ca3d9)
> >   CPU time: 139.686
> > 
> > Note that the benchmarks are run with --enable-optimization.
> > 
> > Let's try to get back to the speed we had before or at least find out
> > exactly what takes time. It might be that computing the nonzero
> > pattern is what takes more time, but that this is necessary to reduce
> > the memory usage.
> > 
> > /Anders
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> > 
> 
> 
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