With the recent interest in Software Defined Networking, many OpenFlow controllers have been released for research and commercial use. However, little public knowledge exists about the architectural choices that allow one controller to outperform another in production environments. In this paper, we aim to identify key performance bottlenecks and good architectural choices for designing OpenFlow-based SDN controllers. With this aim in mind, we evaluate the performances of four prominent open-source OpenFlow controllers: NOX [1], Beacon [2], Maestro [3] and Floodlight [4]. Since these controllers support multi-threading, we deploy them on shared memory multicore machines and benchmark their key architectural components under different metrics including thread scalability, switch scalability and latency in a custom cluster testbed. Our results lead to important architectural guidelines that can be used to improve the scalability of existing controllers or to design new ones. We follow these guidelines to implement an OpenFlow controller which outperforms existing controllers on assorted scalability metrics.