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Caching Characteristics Caching Characteristics of Internet and Intranet of Internet and Intranet Web Proxy TracesWeb Proxy Traces

Arthur GoldbergIlya PevznerRobert Buff

Courant Institute of Mathematical SciencesNew York University

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Clients, Servers and ProxyClients, Servers and Proxy

C1

CM

PC2

S1

SN

S2

Clients Ci configured to use cachingproxy P to access N servers Si . Aserver may use a proxy itself.

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HTTP Through a ProxyHTTP Through a ProxyBrowser Proxy Server

Miss

Hit

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Potential Web Caching Potential Web Caching BenefitsBenefits

• Reduce response time by delivering Reduce response time by delivering document from a closer and/or less document from a closer and/or less loaded server than the origin serverloaded server than the origin server

• Save bandwidth costs between Save bandwidth costs between proxy and origin serverproxy and origin server

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GoalsGoals

• Study large internet and intranet Study large internet and intranet tracestraces

• Evaluate caching opportunities and Evaluate caching opportunities and problemsproblems

• Examine cache size needs and Examine cache size needs and document residence timesdocument residence times

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Part 1Part 1

Proxy trace sources Proxy trace sources and proxy and proxy configurationsconfigurations

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Data SourcesData Sources

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ISP UsageISP Usage

• 450,000 users450,000 users

• LoadLoad– PeakPeak

• 500 unique clients500 unique clients

• 30 requests per second30 requests per second

– AverageAverage• 1M requests per day1M requests per day

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ISP hardware detailsISP hardware details

• IBM RS/6000 systemIBM RS/6000 system

• 256 MB RAM256 MB RAM

• Three 4 GB disksThree 4 GB disks

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ISP proxy configuration ISP proxy configuration detailsdetails• 8 proxies nationwide8 proxies nationwide• Netscape 2.5 proxyNetscape 2.5 proxy• 5.5 GB cache size5.5 GB cache size• Netscape extended-2 log formatNetscape extended-2 log format• ParametersParameters

– max-uncheck - 6 hoursmax-uncheck - 6 hours– lm-factor - 0.1lm-factor - 0.1– term-percent - 80%term-percent - 80%

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Intranet UsageIntranet Usage

• 8,000 employees8,000 employees

• LoadLoad– PeakPeak

• VariesVaries

– AverageAverage• 500K requests per day, over 10 hours500K requests per day, over 10 hours

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Intranet hardware detailsIntranet hardware details

• Sun Microsystems Ultra 1 serverSun Microsystems Ultra 1 server

• 1 GB RAM1 GB RAM

• Seven 4 GB disksSeven 4 GB disks

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Intranet proxy Intranet proxy configuration detailsconfiguration details

• 2 proxies2 proxies

• Squid 1.1.21 proxySquid 1.1.21 proxy

• 12 GB disk cache size12 GB disk cache size

• 750MB memory cache size750MB memory cache size

• Extended log formatExtended log format

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Part 2Part 2

Analysis of ISP and Analysis of ISP and Intranet traces Intranet traces assuming unlimited assuming unlimited cache storagecache storage

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Key Cache MetricsKey Cache Metrics

• Hit Ratio (Hit Ratio (HR HR ))

• Fractional Bandwidth Savings (BT)Fractional Bandwidth Savings (BT)

served documents of numbercache the from served documents of number

HR

served bytes of numbercache the in documents withserved bytes of number

BT

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Analyzing Caching Analyzing Caching PropertiesProperties

Hit RateBandwidth savings

HR(ALL) BT(ALL) All

HR(-ACTUAL) BT(-ACTUAL)Cached by operating proxy

Analysis nameDocuments cached

Cachable as per HTTP specification

HR(-RFC) BT(-RFC)

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ISP documents that cannot be ISP documents that cannot be cached, as per HTTP cached, as per HTTP specificationspecification

Reason for non-cacheability

% of entries in ISPtrace

Expires 3.2%

Cache control 0.8%

Pragma: no-cache 0.4%

Request method 0.0%

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Comment about “cookies”Comment about “cookies”• For Prodigy, RFC figures assume that For Prodigy, RFC figures assume that

Netscape proxy follows RFCNetscape proxy follows RFC

• In reality, Netscape proxy does not cache In reality, Netscape proxy does not cache documents with cookiesdocuments with cookies

• Documents with cookies, account for 2% of Documents with cookies, account for 2% of responses in Prodigy trace responses in Prodigy trace

• It follows that RFC figures for Prodigy may be It follows that RFC figures for Prodigy may be up to 2% higher than shownup to 2% higher than shown

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ISP Hit Ratio vs. Trace ISP Hit Ratio vs. Trace Length Length

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0E+00 1.1E+06 2.1E+06 3.2E+06 4.2E+06Trace length

Hit ratio

HR(ALL)

HR(-RFC)

HR(-ACTUAL)

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ISP BT vs. Trace Length ISP BT vs. Trace Length

0

0.2

0.4

0.0E+00 1.3E+06 2.6E+06 3.8E+06 5.1E+06Trace length

Fraction

BT(ALL)

BT(-RFC)

BT(-ACTUAL)

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Intranet HR vs. Trace Intranet HR vs. Trace LengthLength

Hit Ratios

0.00.10.20.30.40.50.60.70.80.91.0

0.0E+00 4.0E+05 8.0E+05 1.2E+06 1.6E+06Trace length

Hit ratio

HR(ALL)

HR(-RFC)

HR(-ACTUAL)

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Intranet BT vs. Trace Intranet BT vs. Trace Length Length

Fractions of Bytes Transferred Saved by Caching

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0E+00 4.0E+05 8.0E+05 1.2E+06 1.6E+06 Trace length

Fraction

BT(ALL)

BT(-RFC)

BT(-ACTUAL)

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Part 3Part 3

Analysis of ISP trace with finite Analysis of ISP trace with finite cache sizes.cache sizes.

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Prophetic Cache Prophetic Cache Replacement AlgorithmReplacement Algorithm

• A A Prophetic Prophetic cache stores exactly the set cache stores exactly the set of documents that will be referenced in of documents that will be referenced in the futurethe future

• An on-line prophetic cache algorithm An on-line prophetic cache algorithm cannot be builtcannot be built

• However, given a trace, prophetic However, given a trace, prophetic caching decisions can be determined off-caching decisions can be determined off-lineline

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Prophetic Cache Prophetic Cache Replacement Algorithm Replacement Algorithm (continued)(continued)

• Cache space used by a prophetic Cache space used by a prophetic cache is the minimum size needed cache is the minimum size needed to avoid cache missesto avoid cache misses– notes:notes:

• true for any maximum residence timetrue for any maximum residence time

• analyses make cyclical tracesanalyses make cyclical traces

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Maximum Hit Rate as a Maximum Hit Rate as a function of residence timefunction of residence time

20 40 60 80 100 120 140Residence time (hours)

0.10.20.30.40.50.6

Hit Rate

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Maximum Hit Rate as a Maximum Hit Rate as a function of residence function of residence time, by document sizetime, by document size

20 40 60 80 100 120 140Residence time (hours)

0.10.20.30.40.50.6

Hit Rate

101 – 1K

1K – 10K 10K – 100K

100K – 1M

1 - 10

11 - 100

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ConclusionsConclusions

• We analyze very long Web proxy traces We analyze very long Web proxy traces from an ISP and an intranetfrom an ISP and an intranet

• We propose a new method to evaluate We propose a new method to evaluate a proxy by comparing the actual hit rate a proxy by comparing the actual hit rate with potential hit ratewith potential hit rate

• We show that it is important to keep the We show that it is important to keep the cache residence time above one daycache residence time above one day

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AddressesAddresses

• E-mail: E-mail: {artg,pevzner,buff}@cs.nyu.edu{artg,pevzner,buff}@cs.nyu.edu

• WWW: WWW: www.cs.nyu.edu/{artg,pevzner,buff}www.cs.nyu.edu/{artg,pevzner,buff}

• Paper and presentation is available Paper and presentation is available at www.cs.nyu.edu/artgat www.cs.nyu.edu/artg