A Conversation with John Hennessy and David Patterson

KUNLE OLUKOTUN: There are many other issues beyond performance that could impact computer architecture. What ideas are there in the architecture realm, and what sort of impact are these other nonperformance metrics going to have on computing?
JOHN HENNESSY: Well, power is easy. Power is performance. Completely interchangeable. How do you achieve a level of improved efficiency in the amount of power you use? If I can improve performance per watt, I can add more power and be assured of getting more performance.
DAVID PATTERSON: It’s something that has been ignored so far, at least in the data center.
JOHN HENNESSY: I agree with that. What happened is we convinced ourselves that we were on a long-term road with respect to ILP that didn’t have a conceivable end, ignoring the fact that with every step on the road we were achieving lower levels of efficiency and hence bringing the end of that road closer and closer. Clearly, issues of reliability matter a lot, but as the work at Berkeley and other places has shown, it’s a far more complicated metric than just looking at a simple notion of processor reliability.
DAVID PATTERSON: Yes, I guess what you’re saying is, performance per watt is still a quantitative and benchmarkable goal.