For cluster integration and grid computing that bridges organizational network boundaries, potential showstoppers include security policies, firewalls and non-routable IP addresses. Virtual Private Network (VPN) technology can overcome these hurdles, but what are the performance implications?

In this paper, we analyze the performance implications of using the open-source OpenVPN software. Our measurements include NetIO and the NAS parallel benchmarks. The main performance implications are additional latency (0.5 milliseconds) and potentially maxed-out CPU load on the VPN gateways. Effective bandwidth and the execution time of applications that run across VPN-connected clusters was affected only if both (1) the wide-area link is fast (above 83 Mbits/s, given our 1.4 GHz Athlon CPU) and (2) the application is communication-intensive.

Moreover, when moving an application from running within a single cluster to running across multiple clusters, we show that besides potential "VPN slowdown" there is "hotspot slowdown": the grid topology can cause traffic hotspots on the inter-cluster links. The more nodes the cross-cluster application runs on, the worse the potential traffic hotspot.