Caching the Internet: A View from a Global Multi-tenant CDN

Abstract

Commercial Content Delivery Networks (CDNs) employ a variety of caching policies to achieve fast and reliable delivery in multi-tenant environments with highly variable workloads. In this paper, we explore the efficacy of popular caching policies in a large-scale, global, multi-tenant CDN. We examine the client behaviors observed in a network of over 125 high-capacity Points of Presence (PoPs). Using production data from the Edgecast CDN, we show that for such a large-scale and diverse use case, simpler caching policies dominate. We find that LRU offers the best compromise between hit-rate and disk I/O, providing 60 fewer writes than FIFO, while maintaining high hit-rates. We further observe that at disk sizes used in a large-scale CDN, LRU performs on par with complex polices like S4LRU. We further examine deterministic and probabilistic cache admission policies and quantify their trade-offs between hit-rate and origin traffic. Moreover, we explore the behavior of caches at multiple layers of the CDN and provide recommendations to reduce connections passing through the system’s load balancers by approximately 50%.