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When the convergence graph of GMRES shows a slow down every 50 iterations, the Number of iteration before restart parameter (default value of 50) should be increased — doubled for example. This may also increase the memory consumption.
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Increasing the Number of iteration in the Multigrid settings, and in the presmoother and postsmoother nodes improves the quality of the preconditioner and convergence of GMRES.
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Since an excessive difference between two multigrid levels can affect the convergence, lowering the Mesh coarsening factor in the Multigrid settings can help convergence.
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Choosing the Right Linear System Solver in the COMSOL Multiphysics Reference Manual
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