To add processing power under power constraints, emerging heterogeneous processors include fast and slow cores on the same chip. This paper demonstrates that this heterogeneity is well suited to interactive data center workloads (e.g., web search, online gaming, and financial trading) by observing and exploiting two workload properties. These workloads may trade response quality for responsiveness. The request service demand is unknown and varies widely with both short and long requests. Subject to per-server power constraints, traditional homogeneous processors either include a few high-power fast cores that deliver high quality responses or many low-power slow cores that deliver high throughput, but not both.