dolfin team mailing list archive

[Jed Brown <jed@xxxxxxxx>]

On Sun 2008-05-18 22:55, Johan Hoffman wrote:
> > On Sun 2008-05-18 21:54, Johan Hoffman wrote:
> >> > On Sat, May 17, 2008 at 04:40:48PM +0200, Johan Hoffman wrote:
> >> >
> >> > 1. Solve time may dominate assemble anyway so that's where we should
> >> > optimize.
> >>
> >> Yes, there may be such cases, in particular for simple forms (Laplace
> >> equation etc.). For more complex forms with more terms and coefficients,
> >> assembly typically dominates, from what I have seen. This is the case
> >> for
> >> the flow problems of Murtazo for example.
> >
> > This probably depends if you use are using a projection method.  If you
> > are
> > solving the saddle point problem, you can forget about assembly time.
> 
> Well, this is not what we see. I agree that this is what you would like,
> but this is not the case now. That is why we are now focusing on the
> assembly bottleneck.
> 
> But
> > optimizing the solve is all about constructing a good preconditioner.  If
> > the
> > operator is elliptic then AMG should work well and you don't have to
> > think, but
> > if it is indefinite all bets are off.  I think we can build saddle point
> > preconditioners now by writing some funny-looking mixed form files, but
> > that
> > could be made easier.
> 
> We use a splitting approach with GMRES for the momentum equation and AMG
> for the continuity equations. This appears to work faitly well. As I said,
> the assembly of the momentum equation is dominating.

Right, you are not solving the saddle point problem.

Jed

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