USITS 2001 Abstract
The Age Penalty and It's Effect on Cache Performance
Edith Cohen, AT&T LabsResearch; and Haim Kaplan, Tel-Aviv University
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.