dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Oct 04, 2011 at 06:21:33PM +0100, Garth N. Wells wrote:
> On 4 October 2011 18:05, Kent-Andre Mardal <kent-and@xxxxxxxxx> wrote:
> >
> >
> > On 4 October 2011 18:13, Anders Logg <logg@xxxxxxxxx> wrote:
> >>
> >> On Tue, Oct 04, 2011 at 04:17:01PM +0100, Garth N. Wells wrote:
> >> > On 4 October 2011 12:24, Anders Logg <logg@xxxxxxxxx> wrote:
> >> > > SystemAssembler does not support subdomains. It is even silently
> >> > > ignoring all other integrals than number 0.
> >> > >
> >> > > This is one of the remaining bugs for 1.0-beta2. I can try to fix it
> >> > > but would like some input on what shape the SystemAssembler is
> >> > > currently in. I haven't touched it that much before since it looks
> >> > > like a bit of code duplication to me. In particular, is it necessary
> >> > > to keep both functions cell_wise_assembly and facet_wise_assembly?
> >> > >
> >> >
> >> > It would require some performance testing to decide. I expect that,
> >> > for performance reasons, both are required.
> >>
> >> I'm getting very strange results. Here are results for assembling
> >> Poisson matrix + vector on a 32 x 32 x 32 unit cube:
> >>
> >>  Regular assembler: 0.658 s
> >>  System assembler:   9.08 s (cell-wise)
> >>  System assembler:    202 s (facet-wise)
> >>
> >> Is this expected?
> >>
> >> What are the arguments against ditching SystemAssembler (for less code
> >> duplication) and adding functionality for symmetric application of BCs
> >> on the linear algebra level?
> >>
> >
> >
> >
> > Earlier system_assemble of A and b was faster than assemble of A and b.
>
> It was faster because it was less general. After being generalised,
> the performance of SystemAssembler was to all intents and purposes the
> same as Assembler.
>
> > Something strange must have happened.
> > SystemAssemble enforce symmetric BC elementwise which
> > is much faster than doing it on linear algebra level.
>
> I'm not sure that that is true. In my experience the difference is marginal.

It is worth testing. At any rate, it would be good if we could do it
also on the linear algebra level (cf for example feature request from
Doug Arnold earlier today).

SystemAssembler is currently lacking support for subdomains and OpenMP
assembly (which I think can be merged into the regular
assembler). It's easier to maintain one single assembler.

To compare the two approaches, we need to (1) implement symmetric
application of bcs and (2) get SystemAssembler to run at normal speed
again. Marie's result is with a version from yesterday so it's not my
bug fixes from today that cause the slowdown.

--
Anders