What is the test for a proportionality challenge to a sentence under the Eighth Amendment?

When considering a proportionality challenge to the length of a sentence under the Eighth Amendment to the United States Constitution, we consider all the circumstances of the case. We begin by comparing the gravity of the offense and the severity of the sentence. In the rare case where this threshold comparison leads to an inference of gross disproportionality, we then compare the defendant's sentence to the sentences received by offenders both in California and other jurisdictions. If this comparison bears out the initial assessment that the sentence is grossly disproportionate, then the sentence is cruel and unusual. (People v. Christensen (2014) 229 Cal.App.4th 781, 805806, 177 Cal.Rptr.3d 712.)