> On Jun 22, 2018, at 4:49 PM, Matthew Knepley <[email protected]> wrote: > >> On Fri, Jun 22, 2018 at 1:47 PM Boyce Griffith <[email protected]> wrote: >> Can you set up the preconditioner so that you can just use GMRES? > > So I think what Boyce is saying is, can't you fix the number of iterates in > the inner Krylov solvers so that it becomes a linear > operator and you can use GMRES?
Yes that is it. I think we default to FGMRES for robustness, because the preconditioner can easily be configured to be nonlinear, but unless you have made some big changes to the algorithm that I think you are using, I think it should be possible to set it up as a stationary linear operator. > Thanks, > > Matt > >>> On Jun 22, 2018, at 4:33 PM, Nishant Nangia <[email protected]> >>> wrote: >>> >>> Hi, >>> >>> I am solving a saddle point system using a shell preconditioner (which >>> itself uses Krylov solvers, hence the use of FGMRES). I had added the >>> option to re-scale parts of the saddle point system to minimize loss of >>> floating point precision for cases where there are varying orders of >>> magnitude in the system/unknowns. >>> >>> I wanted to show that re-scaling can alleviate large differences between >>> the preconditioned and unpreconditioned residual norms. However, I notice >>> that FGMRES only supports right preconditioning, meaning the preconditioned >>> residual is never formed/used (I think). >>> >>> Is there any way to form the preconditioned norm for FGMRES, or does it >>> just not make sense in the context of right-preconditioned iterative >>> solvers? Is there any way to show that the re-scaling is improving the >>> solver convergence (i.e. showing that it ensures that the true and relative >>> residual are close to each other)? >>> >>> Nishant Nangia >>> Northwestern University >>> Ph.D. Candidate | Engineering Sciences and Applied Mathematics >>> Tech L386 > > > -- > What most experimenters take for granted before they begin their experiments > is infinitely more interesting than any results to which their experiments > lead. > -- Norbert Wiener > > https://www.cse.buffalo.edu/~knepley/
