DATE: Thursday, September 8, 2016
TIME: 12:00 pm - 1:00 pm

SPEAKER: Daniel Berger, University of Kaiserslautern, Germany

TITLE: Maximizing Cache Hit Ratios of CDN Memory Caches with Adaptive Size-Aware Admission Control

Most major content providers use content delivery networks (CDNs) to serve web content to their users. CDNs achieve high performance by using a large distributed system of caching servers. The first and fastest caching level in a CDN server is the memory-resident Hot Object Cache (HOC). A major goal of a CDN is to maximize the object hit ratio (OHR) of its HOCs. Maximizing the OHR is challenging because web object sizes are highly variable and HOCs have a small capacity. This challenge has lead to a wealth of sophisticated cache eviction policies. In contrast, cache admission policies have received little attention.

This talk presents AdaptSize: a new HOC caching system based on a size-aware cache admission policy. AdaptSize is based on a new statistical cache tuning method that continuously adapts the parameters of its cache admission policy to the request traffic. In experiments with Akamai production traces, AdaptSize improves the OHR by 30-44% over Nginx and by 49-92% over Varnish, which are two widely-used production systems. Further, AdaptSize's tuning method consistently achieves about 80% of the OHR of offline parameter tuning, and is significantly more robust than state-of-the-art cache tuning methods based on hill climbing. To demonstrate feasibility in a production setting, we show that AdaptSize can be incorporated into Varnish with low processing and memory overheads and negligible impact on cache server throughput.

Daniel S. Berger is a Phd student in computer science at the University of Kaiserslautern, Germany. His research interests intersect systems, mathematical modeling, and performance testing. As part of his PhD work, Daniel is exploring the boundaries of achievable cache hit ratios in Internet content delivery. Daniel has spent several months each year on research visits: at CMU (2015), Warwick University (2014), T-Labs Berlin (2013), ETH Zurich (2012), and at the University of Waterloo (2011). He received his B.Sc (2012) and M.Sc (2014) in computer science from the University of Kaiserslautern. Previously, he worked as a data scientist at the German Cancer Research Center (2008-2010).


  • Ramesh K. Sitaraman (University of Massachusetts at Amherst & Akamai Technologies)
  • Mor Harchol-Balter (Carnegie Mellon University)

VISITOR HOST: Mor Harchol-Balter

Karen Lindenfelser, 86716, or visit