[Dune] [#1018] RFC Patch: allow different convergence criteria in dune-istl

[Dune]

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The following task has a new comment added:

FS#1018 - RFC Patch: allow different convergence criteria in dune-istl
User who did this - Carsten Gräser (Carsten)

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I like the idea of having exchangable criteria but I don't like the proposes solution for the following reasons:

My main concern is that introducing of a general concept of convergence criteria while not having a concept for iterative solvers will not lead to a clean and flexible design. For example the proposed solution is still very restrictive and a general solution would IMHO need access to solver specific data. There is e.g. a criterion for the cg method that estimates |u-u_k|_A using quantities already computed by the cg-step (see Deuflhard/Weiser). Although not being really flexible the proposed solution introduces ten new interface methods.

A flexible criterion class could e.g. get the solver or an IterartionState object as base class pointer and ask it for specific data using a dynamic_cast to derived classes. This would allow to implement generic and solver specific criteria without blowing up the interface.

Since this solution would be far more invasive I do not propose to introduce it now. In order to implement the criterion that you need it might be clearer to just hardwire it additionally instead of introducing a new concept that is not satisfactory in the long run.

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