What is the test for a sentence that violates the Eighth Amendment against cruel and unusual punishment?

A sentence violates the Eighth Amendment proscription against cruel and unusual punishment only if it is grossly disproportionate to the crime. (Graham v. Florida (2010) 560 U.S. 48, 60 [176 L.Ed.2d 825].) "A court must begin by comparing the gravity of the offense and the severity of the sentence. [Citation.] '[I]n the rare case in which [this] threshold comparison . . . leads to an inference of gross disproportionality' the court should then compare the defendant's sentence with the sentences received by other offenders in the same jurisdiction and with the sentences imposed for the same crime in other jurisdictions." (Ibid.)