The age penalty and its effect on cache performance

Web content caching is recognized as an effective mechanism to decrease server load, network traffic, and user-perceived latency. An HTTP compliant cache associates with each cached object an expiration time calculated according to directives set by the object's origin server. The cache incurs a miss when it has no cached copy of a requested object or when the existing copy had expired (is not fresh). Upon a miss, the cache needs to fetch or validate a copy through exchanges with another cache with a fresh copy or the origin server. Thus, misses generate traffic and prolong service times.

Caches are deployed as proxies, reverse proxies, and hierarchically and as a result, caches often serve other caches. As this happens, content age at higher-level caches, in addition to availability and freshness, emerges as a performance factor. The age of a cached copy of an object is the elapsed time since fetched from the respective origin. Fresh cached copies of the same object can have different ages and older copies typically expire sooner. Therefore, a proxy cache would suffer a higher miss rate if it receives older objects (e.g., from a reverse-proxy cache). Similarly, reverse-proxy caches that serve proxy-caches receive more requests than an origin server would have received. We refer to the increase in miss rate due to age as the age penalty. We use trace-based simulations to measure the extent of the age penalty for content served by content delivery networks and large caches. Even though the age penalty had not been considered previously, we demonstrate that it can be significant, and moreover, can highly vary under different practices.