According to the Supreme Court, the Eighth Amendment forbids some punishments entirely, and forbids some other punishments that are excessive when compared to the crime, or compared to the competence of the perpetrator.

Answer this question: when may a non-capital sentence be considered to violate the 8th amendment? Let's give stream of consciousness first:
This is the relevant information: According to the Supreme Court, the Eighth Amendment forbids some punishments entirely, and forbids some other punishments that are excessive when compared to the crime, or compared to the competence of the perpetrator.
So, the final answer is excessive when compared to the crime , or compared to the competence of the perpetrator.