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The Age Penalty and its effect on cache performance

Edith Cohen

AT&T Labs-Research

Haim Kaplan

Tel-Aviv University

Presenting: Edith Cohen

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Distribution of Web Content

Origin server

cache cache

•Content of URL can be modified by the origin. •Copies are cached throughout

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HTTP Freshness Control

• 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.

Body(content)

headerCache-directives

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HTTP Cache Serving a Request

•No cached copy GET a fresh copy •Stale cached copy If-Modified-Since GET a fresh copy

•“Not-Modified” update header •“Modified” update content and header

•Fresh cached copy

GET www.cnn.com/WEATHER/

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“hits” and “misses”

hit-rate metric: c-hit/(c-hit+c-miss)latency(c-miss) > latency(f-miss) >> latency(f-

hit) “hit-rate” does not capture freshness

freshness-rate: f-hit/c-hit

f-hit f-miss c-miss

remote RTTs X X

content-transfer X

“traditional”: c-hit (hit) (miss)

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“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

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Origin Server to User

www.cnn.com

reverse proxy

reverse proxy

reverse proxy

proxy cache

proxy cache

AS

AS

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Issues for Cascaded Caches

• Path between Web server and end-user often includes 2 or more caches.

• downstream cache sends older copies more misses at upstream caches– more traffic between caches

– increased user-perceived latency

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1:00pm

Freshness-lifetime= 10 HAge = 0

Distributing Copies

Origin server

downstream cache upstream cache

7:00pm

Freshness-lifetime= 10 HAge = 6 H

Freshness-lifetime= 10 HAge = 9:59 H

10:59pm

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Modeling Sources of a Cache

•“Cache” misses are forwarded to a cache

Cache-2

www.cnn.com

Cache

Cache-1

•“Origin” misses are forwarded to authoritative (origin) server.

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Source Models…

•origin: age(t) = 0 •cache: age(t) = T - (t+a) mod T

Object with freshness-lifetime = T. The age of copy retrieved at time t is:

Theorem: miss_origin(S) < miss_cache(S) < 2*miss_origin(S)

Request sequence S

Age-Penalty)(

)()()(

Sfrate

SfrateSfrateS

cache

cacheorigin

Definition:

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…Age-penalty

Requests:

cache

T

t

TTLorigin

m f-mhm f-m f-m

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Trace-driven Simulations

log Origin Cache penalty

NLANR UC 52% 43% 21%

NLANR SD 47% 38% 24%

freshness-rate for different configurations

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CDN server

CDN server

CDN server

Content Delivery Network

www.cnn.com

AS

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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.

CDNwww.cnn.com Cache

•HTTP freshness control•Proprietary freshness control

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CDN Age-Handling PracticesPractice: “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?

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…Implications of Intact Headers

Age = time on CDN server copies often served with 0 TTL!

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…Implications of Intact Headers

CDN NLANR log

%requests

Freshness-rate

CDN origin

Sandpiper UC 0.4% 5% 76%

Sandpiper SD 0.5% 6% 67%

Akamai UC 1.7% 5% 61%

Akamai SD 1.1% 6% 63%

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…..Equating Lifetimes

How much can be gained?

We estimate about 20% of requeststo CDN (Akamai) servers would beeliminated.

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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...

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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 (!!!)…