KSPFGMRES#

Implements the Flexible Generalized Minimal Residual method. Flexible Krylov Methods

Options Database Keys#

-ksp_gmres_restart - the number of Krylov directions to orthogonalize against
-ksp_gmres_haptol - sets the tolerance for “happy ending” (exact convergence)
-ksp_gmres_preallocate - preallocate all the Krylov search directions initially (otherwise groups of vectors are allocated as needed)
-ksp_gmres_classicalgramschmidt - use classical (unmodified) Gram-Schmidt to orthogonalize against the Krylov space (fast) (the default)
-ksp_gmres_modifiedgramschmidt - use modified Gram-Schmidt in the orthogonalization (more stable, but slower)
-ksp_gmres_cgs_refinement_type <refine_never,refine_ifneeded,refine_always> - determine if iterative refinement is used to increase the stability of the classical Gram-Schmidt orthogonalization.
-ksp_gmres_krylov_monitor - plot the Krylov space generated
-ksp_fgmres_modifypcnochange - do not change the preconditioner between iterations
-ksp_fgmres_modifypcksp - modify the preconditioner using KSPFGMRESModifyPCKSP()

Notes#

See KSPFGMRESSetModifyPC() for how to vary the preconditioner between iterations

Only right preconditioning is supported.

The following options -ksp_type fgmres -pc_type ksp -ksp_ksp_type bcgs -ksp_view -ksp_pc_type jacobi make the preconditioner (or inner solver) be bi-CG-stab with a preconditioner of Jacobi.