What is the test for a sentence that violates the state prohibition against cruel and unusual punishment?

A sentence violates the state prohibition against cruel and unusual punishment if " 'it is so disproportionate to the crime for which it is inflicted that it shocks the conscience and offends fundamental notions of human dignity.' " (People v. Dillon (1983) 34 Cal.3d 441, 478.) The federal Constitution " 'forbids only extreme sentences that are "grossly disproportionate" to the crime,' " and "[s]uccessful grossly disproportionate challenges are ' "exceedingly rare" ' and appear only in an ' "extreme" ' case." (People v. Em (2009) 171 Cal.App.4th 964, 977.)