[firedrake] disentangling solver setup costs

[Eike Mueller]

Hi Lawrence,

> 
> So you build UU, UP, PU, and PP separately?

Yes, but as you say, I should only build them once, not multiple times, which is incredibly wasteful.

> 
> Given they never change, I would assemble them up front and then just
> use them as and where.  If you always do (whereever you need it):
> 
> UU = assemble(...)
> 
> Then you will pay the cost multiple times.
> 
> If you build everything up front then calling mat.M a second time
> doesn't reassemble (since now the promise to assemble is fulfilled)
> and the operator is ready.

Ok, good, so that sounds like what I’m going to do will work and I only pay the cost of assembling the mixed operator once.

Thanks,

Eike

> 
> Lawrence
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