usits ‘01 the age penalty and its effect on cache performance edith cohen at&t labs-research...
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USITS ‘01
The Age Penalty and its effect on cache performance
Edith Cohen
AT&T Labs-Research
Haim Kaplan
Tel-Aviv University
Presenting: Edith Cohen
USITS ‘01
Distribution of Web Content
Origin server
cache cache
•Content of URL can be modified by the origin. •Copies are cached throughout
USITS ‘01
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 (!!!)…
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