What is the test for gross disproportionality under the Eighth Amendment?

mistaken. Under a federal constitutional analysis, "'[t]he Eighth Amendment, which forbids cruel and unusual punishments, contains a "narrow proportionality principle" that "applies to noncapital sentences."' [Citation.] The appropriate standard for determining whether a particular sentence for a term of years violates the Eighth Amendment is gross disproportionality. That is, '[t]he Eighth Amendment does not require strict proportionality between crime and sentence. Rather, it forbids only extreme sentences that are "grossly disproportionate" to the crime. [Citations.]' [Citation.] Successful grossly disproportionate challenges are '"exceedingly rare"' and appear only in an '"extreme"' case. [Citation.]" (People v. Em (2009) 171 Cal.App.4th 964, 977 (Em).)