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The Age Penalty and its effect on cache performance

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21 diapositives · 356 Views · Oct 15, 2014

This paper explores the age penalty effect on cache performance concerning web content distribution. It delves into how freshness control directives and the age of cached copies affect cache hits and misses. The study highlights significant findings regarding the latency and effectiveness of cascaded caches, particularly in Content Delivery Networks (CDNs). Results indicate that aging content can lead to a 25% decrease in cache effectiveness. The paper calls for heightened awareness and further research into content-aging practices and their implications for web performance optimization.

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  1. The Age Penaltyand its effect on cache performance Haim Kaplan Tel-Aviv University Edith Cohen AT&T Labs-Research Presenting: Edith Cohen

  2. Distribution of Web Content cache cache Origin server • Content of URL can be modified by the origin. • Copies are cached throughout

  3. HTTP Freshness Control Cache-directives • Cached copies have: • Freshness lifetime • Age (elapsed time since fetched from origin) • TTL (Time to Live) = freshness lifetime – age • Expired copies must be validated before they can be used. header Body (content)

  4. HTTP Cache Serving a Request GET www.cnn.com/WEATHER/ c-miss • No cached copya GET a fresh copy a$ • Stale cached copy a • If-Modified-Since GET a fresh copy • “Not-Modified” a update header a$ • “Modified” a update content and header a$ • Fresh cached copy a$ f-miss c-miss f-hit

  5. freshness-rate: f-hit/c-hit “hits” and “misses” hit-rate metric:c-hit/(c-hit+c-miss) latency(c-miss) > latency(f-miss) >> latency(f-hit) c “hit-rate” does not capture freshness

  6. “Value” of a Cached Copy • Frequency/pattern of requests • Size (storage cost) • Fetching cost (latency/bandwidth) • Modification rate (likelihood of c-miss) • Age (likelihood of f-miss) • Important issues for: • replacement and pre-fetching strategies • cascaded caches

  7. Origin Server to User reverse proxy proxy cache reverse proxy AS AS www.cnn.com proxy cache reverse proxy

  8. Issues for Cascaded Caches • Path between Web server and end-user often includes 2 or more caches. c • downstream cache sends older copies c more misses at upstream caches • more traffic between caches • increased user-perceived latency

  9. 7:00pm 1:00pm 10:59pm Freshness-lifetime= 10 H Age = 0 Freshness-lifetime= 10 H Age = 6 H Freshness-lifetime= 10 H Age = 9:59 H Distributing Copies upstream cache downstream cache Origin server

  10. “Origin”misses are forwarded to authoritative (origin) server. • “Cache” misses are forwarded to a cache www.cnn.com Cache Cache-1 Cache-2 Modeling Sources of a Cache

  11. Theorem:miss_origin(S)<miss_cache(S)< 2*miss_origin(S) Object with freshness-lifetime = T. The age of copy retrieved at time t is: • origin: age(t) = 0 • cache: age(t) = T - (t+a) mod T Definition: Age-Penalty Source Models… Request sequence S

  12. TTL cache origin T t Requests: m f-m f-m m h f-m …Age-penalty

  13. Trace-driven Simulations freshness-rate for different configurations

  14. CDN server CDN server CDN server Content Delivery Network AS www.cnn.com

  15. HTTP freshness control • Proprietary freshness control CDN www.cnn.com Cache Content Delivery Networks • CDN servers are a system of reverse proxy caches, placed throughout the Internet. • Proprietary, non-HTTP, freshness control between origin and CDN servers, allowing for longer TTL durations.

  16. CDN Age-Handling Practices Practice: “Intact headers” CDN servers act as a cache, end-to-end headers are intact. (-3/2000). What are the implications? Practice: “emulate origin” CDN servers rewrite age to 0 (3/2000-present) Practice: “equate lifetime” CDN servers rewrite HTTP freshness lifetime is equated (extended) to “proprietary” freshness lifetime. How much can be gained?

  17. Age = time on CDN serverc copies often served with 0 TTL! …Implications of Intact Headers

  18. …Implications of Intact Headers

  19. …..Equating Lifetimes How much can be gained? We estimate about 20% of requests to CDN (Akamai) servers would be eliminated.

  20. Conclusion • Content-aging can have significant performance effect on cascaded Web caches (25% decrease in effectiveness of cache hits). • Aging effects are not being carefully accounted for (see CDN practices). • Content-aging warrants more awareness by practitioners, and more research...

  21. Follow-up Work • More “source” types (e.g., alternating between several downstream caches) • Pre-term refreshes by downstream cache (sporadic updates by downstream cache may decrease performance, but this can be avoided) • Longer lifetime durations at upstream caches (tradeoff of staleness and latency) • Rejuvenation by downstream caches (refresh “fresh” popular copies to set their age to 0). Proper use can improve performance, but otherwise can decrease performance (!!!)…