Adjusting solver preconditioner tolerances#

To solve the Stokes equations it is necessary to lower the condition number of the Stokes matrix by preconditioning it. In ASPECT a right preconditioner

\[\begin{split}Y^{-1} = \begin{pmatrix} \widetilde{A^{-1}} & -\widetilde{A^{-1}}B^{T}\widetilde{S^{-1}} \\ 0 & \widetilde{S^{-1}} \end{pmatrix}\end{split}\]

is used to precondition the system, where \(\widetilde{A^{-1}}\) is the approximate inverse of the A block and \(\widetilde{S^{-1}}\) is the approximate inverse of the Schur complement matrix. Matrix \(\widetilde{A^{-1}}\) and \(\widetilde{S^{-1}}\) are calculated through a CG solve, which requires a tolerance to be set. In comparison with the solver tolerances of the previous section, these parameters are relatively safe to use, since they only change the preconditioner, but can speed up or slow down solving the Stokes system considerably.

In practice \(\widetilde{A^{-1}}\) takes by far the most time to compute, but is also very important in conditioning the system. The accuracy of the computation of \(\widetilde{A^{-1}}\) is controlled by the parameter Linear solver A block tolerance which has a default value of \(1e-2\). Setting this tolerance to a less strict value will result in more outer iterations, since the preconditioner is not as good, but the amount of time to compute \(\widetilde{A^{-1}}\) can drop significantly resulting in a reduced total solve time. The cookbook Crustal deformation for example can be computed much faster by setting the Linear solver A block tolerance to \(5e-1\). The calculation of \(\widetilde{S^{-1}}\) is usually much faster and the conditioning of the system is less sensitive to the parameter Linear solver S block tolerance, but for some problems it might be worth it to investigate.