As discussed in Chapter SLES: Linear Equations Solvers , the default linear solvers are
-ksp_type <ksp_name> -pc_type <pc_name>One can also specify a variety of runtime customizations for the solvers, as discussed throughout the manual.
In particular, note that the default restart parameter for GMRES is 30, which may be too small for some large-scale problems. One can alter this parameter with the option -ksp_gmres_restart <restart> or by calling KSPGMRESSetRestart(). Section Krylov Methods gives information on setting alternative GMRES orthogonalization routines, which may provide much better parallel performance.