dolfin team mailing list archive

["Robert C. Kirby" <kirby@xxxxxxxxxxxx>]

> Improving the assembly is interesting, but it would probably be more
> worthwile to get the results for Stokes. (It would also put FFC and
> DOLFIN to the test and perhaps we can nail down some bugs in the
> process.)
I agree that these are the results that are more likely of interest  
to the broader community, and we should push on them.

> Another point is that to improve the speed of the assembly (which I
> think is already pretty good), it would be easier to improve on the
> interaction with the mesh (which we need to redo with Sieve anyway).

I claim that there is something also to be gained in batching  
together computations.
 Currently, for each element you
- get the affine map
- get the dof
- build the matrix
- insert it.

If you loop over N elements and get affine maps stored in a C array,  
then loop over N elements and get the dof in a C array, then build  
matrices for N arrays (this is level 3 blas instead of level 2), then  
insert N matrices, you're likely to improve performance.

Of course, I recall that the geometry and insertion into PETSc are  
the bottlenecks in the process?  However, when you go to Navier- 
Stokes (or any trilinear operator), building the element matrix is  
much more expensive while insertion time stays the say (relative to  
Stokes, not Poisson, since there are more dof).  You'll see the  
effects of level 3 BLAS much more in this regime.  But this is not  
hard to try out and put into DOLFIN.


Rob Kirby

"Mathematical software should be mathematical."