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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