Virtualization and multi-tenancy are attractive paradigms to improve the utilization of computing infrastructures and hence to reduce costs. In order to provide a high degree of resource sharing without sacrificing predictable cloud application performance, strict performance isolation needs to be ensured. This is non-trivial and requires models which account for all components where applications may interfere: similarly to security, the predictability of cloud application performance can only be as good as the least predictable component in the model. This paper identifies a new source of potential performance interference that has been overlooked so far: the network hypervisor — a critical component in any multi-tenant network. We present a first measurement study of the performance implications of the network hypervisor in Software-Defined Networks (SDNs). For the purpose of our study, we developed a new open-source benchmarking tool for OpenFlow control and data planes. We show that cloud application performance may appear unpredictable if the network hypervisor is not accounted for: the performance does not only depend on the specific hypervisor implementation and workload (e.g., OpenFlow message types), but also on the number of tenants and the size of the network. Hence, our results suggest that hypervisors should be included in our performance models, and their performance benchmarked and compared similarly to other crucial software components such as the SDN controller.

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