[petsc-dev] Right-preconditioned GMRES

Jed Brown jed at jedbrown.org
Sun Oct 13 14:07:23 CDT 2019

"Smith, Barry F. via petsc-dev" <petsc-dev at mcs.anl.gov> writes:

>   Is this one process with one subdomain? (And hence no meaningful overlap since there is nothing to overlap?) And you expect to get the "exact" answer on one iteration? 
>   Please run the right preconditioned GMRES with -pc_asm_type [restrict and basic and none]  -ksp_monitor_true_solution  and send the output for the three cases.
>   For kicks you can also try FGMRES (which always uses right preconditioning) to see if the same problem appears.

Note that FGMRES (and GCR) does not need to apply the preconditioner
again to recover the solution, thus you save one preconditioner
application by choosing them (in exchange for needing to store two
vectors per Krylov iteration).  This often pays off when the iteration
count is <10 or so.  It's also a reason why results may be different,
e.g., in case of null space issues.

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