dolfin team mailing list archive

["Johan Hoffman" <jhoffman@xxxxxxxxxx>]

> 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
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