> 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.
We are solving the saddle point problem with an outer fixpoint iteration (or Newton iteration), where the GMRES-AMG splitting can be seen as a preconditioner. /Johan > Jed > _______________________________________________ > DOLFIN-dev mailing list > [email protected] > http://www.fenics.org/mailman/listinfo/dolfin-dev > _______________________________________________ DOLFIN-dev mailing list [email protected] http://www.fenics.org/mailman/listinfo/dolfin-dev
