  Distributed multi-controller deployment is a promising method to achieve a
scalable and reliable control plane of Software-Defined Networking (SDN).
However, it brings a new challenge for balancing loads on the distributed
controllers as the network traffic dynamically changes. The unbalanced load
distribution on the controllers will increase response delay for processing
flows and reduce the controllers'throughput. Switch migration is an effective
approach to solve the problem. However, existing schemes focus only on the load
balancing performance but ignore migration efficiency, which may result in high
migration costs and unnecessary control overheads. This paper proposes
Efficiency-Aware Switch Migration (EASM) to balance the controllers'loads and
improve migration efficiency. We introduce load difference matrix and trigger
factor to measure load balancing on controllers. We also introduce the
migration efficiency problem, which considers load balancing rate and migration
cost simultaneously to optimally migrate switches. We propose EASM to
efficiently solve to the problem. The simulation results show that EASM
outperforms baseline schemes by reducing the controller response time by about
21.9%, improving the controller throughput by 30.4% on average, maintaining
good load balancing rate, low migration costs and migration time, when the
network scale changes.
