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You can specify the size of the stored subspace between nonlinear or parametric iterations k (where k < GMRES restart length) can be set using the Number of approximate eigenvectors field (default: 25).
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Use the options in the Reuse subspace list to control if the subspace is stored or deleted between iterations. If you choose Off, the subspace is always deleted. This means that GMRES will be called again in the next nonlinear or parametric iteration. If you choose On, the subspace is always stored. By default, the Automatic storage is selected, and it means that the subspace is deleted if you get quick convergence, because in that case, GMRES can be expected to converge quickly in the next nonlinear or parametric iteration.
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In the Minimum number of restarts field, which is available when Reuse subspace is set to Automatic, you can specify a minimum value for the number of restarts (default: 5). Using this setting can be useful to control when the subspace should be cleared between solver calls in cases when a subspace exists between solver calls — for instance, in a frequency sweep. The GCRO-DR method might then requires more cycles than usual to converge.
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Select the Use relative subspace size check box to force the subspace storage whenever GMRES converges in more than k iterations (set in the Number of approximate eigenvectors field). In that case, the size of the stored subspace is equal to the fraction of GMRES iterations controlled by the value (larger than 0, maximum 1) in the Fraction of GMRES iterations field.
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The default is Automatic, meaning that the main solver is responsible for error management. The solver checks for errors for every linear system that is solved. To avoid false termination, the main solver continues iterating until the error check passes or until the step size is smaller than about 2.2·10−14. With this setting, linear solver errors are either added to the error description if the nonlinear solver does not converge, or added as a warning if the errors persist for the converged solution. The nonlinear solver continues when it reaches the maximum number of iterations. Use the Maximum number of intermediate iterations field to control the efforts spent in each linear solver call (default: 100 iterations).
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Choose Yes to check for errors for every linear system that is solved. If an error occurs in the main solver, warnings originating from the error checking in the direct solver appear. The error check asserts that the relative error times a stability constant ρ is sufficiently small. This setting is useful for debugging problems with singular or near singular formulations.
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Choose No for no error checking.
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For an example using a Stationary Solver, see Micromixer: Application Library path COMSOL_Multiphysics/Fluid_Dynamics/micromixer.
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