[firedrake] disentangling solver setup costs

[Eike Mueller]

Hi Lawrence,

it looks like I’m setting up the same operator multiple times in my code, so this is very wasteful and explains why my setup costs are so high.

Basically I need four assembled operators which describe the 2x2 matrix of the velocity-pressure system in several parts of my code. What happens if I construct those centrally, but then use them in different parts of the code? For example, I might want to apply the UU,UP,PU and PP-operators when I apply the mixed system matrix in the outer loop of the iterative solver (and I would do this in my MixedOperator class). For this I would extract the handles with .M.handle as below. But somewhere else I need to solve for the velocity mass matrix (I would do this in the class Mutilde), so I would throw the same UU operator into a LinearSolver class. Would the operator be built multiple times in this case? Or can I call mat.M once, and this will then be the only time where the operator gets assembled? If that is the case, then I need some class which holds the UU,UP,PU and PP operators and then feed them to the other classes appropriately.

Thanks,

Eike

> On 21 Apr 2015, at 11:23, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> wrote:
> 
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> On 21/04/15 11:20, Eike Mueller wrote:
>> Hi Lawrence,
>> 
>> thanks! When I build a LinearSolver, can I solve the same problem
>> (but for different RHSs) to two different tolerances? Currently I
>> construct the LinearSolver twice, and that is very wasteful since I
>> assemble the matrix twice).
> 
> Yes:
> 
> foo = LinearSolver(..., solver_parameters={'ksp_rtol': 1e-10})
> 
> foo.solve(...)
> 
> foo.parameters = {'ksp_rtol': 1e-4}
> 
> Note that setting this dict completely overwrites all the solver options.
> 
> Lawrence
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