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Stabilize the CG solver

Nicolas Tancogne-Dejeanrequested to merge
stabilize_cg_solver into main

Description

Stabilize the CG linear solver

Analyzes with valgrind+verrou shows that the application of the CG solver quickly accumulate round-off errors leading to completely erratic results, for slow problems like the Poisson equation.

Here a simple restarting solution is implemented, which stabilizes well th algorithm, making the results much more robbust to round-off error, at only a mild cost (one extra operator application every 50 iterations).

News snippet

Numerical stabilization of the internal linear CG solver

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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