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An Integrated Approach to Improving Web Performance

Lili Qiu

Cornell University

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Outline Motivation & Open Issues Solutions

Study the workload of a busy Web server Optimize TCP performance for Web

transfers Provision the content distribution networks

Summary & Other Work

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Motivation Web is the most dominant traffic in the

Internet today Accounts for over 70% wide-area traffic

Web performance is often unsatisfactory WWW – World Wide Wait Consequence: losing potential

customers! Network congestio

nOverloadedWeb server

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Challenges in Providing Highly Efficient Web Services

Workload characterization The workload of busy Web

sites is not well understood

Protocol inefficiency Mismatch between Web

transfers and TCP protocol Infrastructure provisioning

Current trend: Content Distribution Networks

Problem: Where to place replicas?

WorkloadCharacterization

ProtocolInefficiency

InfrastructureProvisioning

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Our Solutions Web Workload Characterization

Study the workload of a busy Web server Improve protocol efficiency

Optimize TCP startup performance for Web transfers

Provision Web replication infrastructure Develop placement algorithms for content

distribution networks (CDNs)

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Part I Web Workload Characterization

The Content and Access Dynamics of a Busy Web Site: Findings and Implications. Proceedings of ACM SIGCOMM 2000, Stockholm, Sweden, August 2000. (Joint work with V. N. Padmanabhan)

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Motivation Solid understanding of Web workload is critical

for designing robust and scalable systems Missing piece in previous work: workload of

busy Web servers

Internetreplica

proxy

replica

proxy

proxy

Clients Servers

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Overview MSNBC server site

a large news site consistently ranked among the busiest sites in the Web server cluster with 40 nodes 25 million accesses a day (HTML content alone) Period studied: Aug. – Oct. 99 & Dec. 17, 98 flash crowd

Server logs HTTP access logs Content Replication System (CRS) logs HTML content logs

Data analysis Content dynamics Access dynamics

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Temporal Stability of File Popularity

Methodology Consider the traces from

a pair of days Pick the top n popular

documents from each day Compute the overlap

Results One day apart:significant

overlap (80%) Two months apart:

smaller overlap (20-80%) Ten months apart: very

small overlap (mostly below 20%)

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0.2

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0.6

0.8

1

1 10 100 1000 10000 100000

# popular documents picked

Exte

nt o

f ove

rlap

17DEC98 - 18OCT99 01AUG99 - 18OCT99 17OCT99 - 18OCT99

The set of popular documents remains stable for days

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Spatial Locality inClient Accesses

Normal Day

0

0.2

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0.8

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0 10000 20000 30000 40000 50000

Domain ID

Frac

tion

of re

ques

ts s

hare

d

Domain membership is significant except when there is a “hot” event of global interest

Dec. 17, 1998

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0.6

0.8

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1.2

0 5000 10000 15000 20000 25000 30000 35000

Domain IDFr

actio

n of

requ

ests

sha

red

Trace

Random

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Spatial Distribution of Client Accesses

Cluster clients using network aware clustering [KW00]

IP addresses with the same address prefix belongs to a cluster

Top 10, 100, 1000, 3000 clusters account for about 24%, 45%, 78%, and 94% of the requests respectively

A small number of client clusters contribute most of the requests.

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The Applicability of Zipf-law to Web requests

The Web requests follow Zipf-like distribution Request frequency 1/i, where i is a document’s ranking

The value of is much larger in MSNBC traces 1.4 – 1.8 in MSNBC traces smaller or close to 1 in the proxy traces close to 1 in the small departmental server logs [ABC+96] Highest when there is a hot event

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0.5

1

1.5

2

MSNBC Proxies Less popular servers

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Impact of larger Accesses in MSNBC traces

are much more concentrated90% of the accesses are accounted by

Top 2-4% files in MSNBC traces

Top 36% files in proxy traces (Microsoft proxies and the proxies studied in [BCF+99])

Top 10% files in small departmental server logs reported in [AW96]

Popular news sites like MSNBC see much more concentrated accesses Reverse caching and replication can be very

effective!

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0.2

0.4

0.6

0.8

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1.2

0 0.5 1 1.5

Percentage of Documents (sorted by popularity)

Pe

rce

nta

ge

of R

eq

uest

s

12/17/98 Server Traces 08/01/99 Server Traces10/06/99 Proxy Traces

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Part II Transport Layer Optimization for the Web Speeding Up Short Data Transfers: Theory,

Architectural Support, and Simulation Results. Proceedings of NOSSDAV 2000 (Joint work with Yin Zhang and Srinivasan Keshav)

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Motivation Characteristics of Web data transfers

Short & bursty [Mah97] Use TCP

Problem: Short data transfers interact poorly with TCP !

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TCP/Reno Basics

Slow Start Exponential growth in

congestion window, Slow: log(n) round

trips for n segments Congestion

Avoidance Linear probing of BW

Fast Retransmission Triggered by 3

Duplicated ACK’s

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Related Work P-HTTP [PM94]

Reuses a single TCP connection for multiple Web transfers, but still pays slow start penalty

T/TCP [Bra94] Cache connection count, RTT

TCP Control Block Interdependence [Tou97]: Cache cwnd, but large bursts cause losses

Rate Based Pacing [VH97] 4K Initial Window [AFP98] Fast Start [PK98, Pad98]

Need router support to ensure TCP friendliness

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Our Approach Directly enter Congestion Avoidance Choose optimal initial congestion window

A Geometry Problem: Fitting a block to the service rate curve to minimize completion time

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Optimal Initial cwnd Minimize completion time by having the

transfer end at an epoch boundary.

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Shift Optimization Minimize initial cwnd while keeping the

same integer number of RTTs

Before optimization:cwnd = 9

After optimization:cwnd = 5

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Effect of Shift Optimization

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TCP/SPAND Estimate network state by sharing performance

information SPAND: Shared PAssive Network Discovery [SSK97]

Directly enter Congestion Avoidance, starting with the optimal initial cwnd

Avoid large bursts by pacing

Internet

Web Servers

Performancegateway

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Implementation Issues Scope for sharing and aggregation

24-bit heuristic network-aware clustering [KW00]

Collecting performance information Performance reports, New TCP option, Windmill’s

approach, … Information aggregation

Sliding window average Retrieving estimation of network state

Explicit query, active push, … Pacing

Leaky-bucket based pacing

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Opportunity for Sharing MSNBC: 90% requests arrive within 5 minutes

since the most recent request from the same client network (using 24-bit heuristic)

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Cost for Sharing MSNBC: 15,000-25,000 different client

networks in a 5-minute interval during peak hours (using 24-bit heuristic)

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Simulation Results Methodology

Download files in rounds Performance Metric

Average completion time TCP flavors considered

reno-ssr: Reno with slow start restart reno-nssr: Reno w/o slow start restart newreno-ssr: NewReno with slow start restart newreno-nssr: NewReno w/o slow start restart

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Simulation Topologies

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T1 Terrestrial WAN Link withSingle Bottleneck

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T1 Terrestrial WAN Link withMultiple Bottlenecks

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TCP Friendliness

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Summary TCP/SPAND significantly reduces latency

for short data transfers 35-65% compared to reno-ssr / newreno-ssr 20-50% compared to reno-nssr / newreno-

nssr Even higher for fatter pipes

TCP/SPAND is TCP-friendly TCP/SPAND is incrementally deployable

Server-side modification only No modification at client-side

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Part III Provision Content Distribution Networks (CDNs)

On the Placement of Web Server Replicas. To appear in INFOCOM'2001. (Joint work with V. N. Padmanabhan and G. M. Voelker)

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Introduction to CDNs Content providers want to offer

better service to their clients at lower cost

Increasing deployment of content distribution networks (CDNs)

Akamai, Digital Island, Exodus … Idea: a network of servers Features:

Outsourcing infrastructure Improve performance by moving

content closer to end users Flash crowd protection

CDNserver

server

ClientsContent

Providers

server

server

server

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Placement of CDN servers Goal

minimize users’ latency or bandwidth usage

Minimum K-median problem

Select K centers to minimize the sum of assignment costs

Cost can be latency or bandwidth or other metric we want to optimize

NP-hard problem

CDNserver

server

server

server

server

ClientsContent

Providers

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Placement Algorithms Tree based algorithm [LGG+99]

Assume the underlying topologies are trees, and model it as a dynamic programming problem

O(N3M2) for choosing M replicas among N potential places

Random Pick the best among several random

assignments Hot spot

Place replicas near the clients that generate the largest load

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Placement Algorithms (Cont.)

Greedy algorithmGreedy(N,M) { for I = 1 .. M { for each remaining replica R {

cost[R] = cost after placing an additional replica at R

} select the replica with the lowest cost }}

Super Optimal algorithm Lagrangian relaxation + subgradient method

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Simulation Methodology Network topology

Randomly generated topologies Using GT-ITM Internet topology generator

Real Internet network topology AS level topology obtained using BGP routing data from

a set of seven geographically dispersed BGP peers Web Workload

Real server traces MSNBC, ClarkNet, NASA Kennedy Space Center

Performance Metric Relative performance: costpractical/costsuper-optimal

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Simulation Results inRandom Graph Topologies

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Simulation Results inReal Internet Topologies

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Effects of Imperfect Knowledge about Input Data

Predict load using moving window average

(a) Perfect knowledge about topology

(b) Knowledge about Topology with a factor of 2

accurate

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Summary First experimental study on placement of CDNs Knowledge about client workload and topology is

crucial for provisioning CDNs The greedy algorithm performs the best

Within a factor of 1.1 – 1.5 of super-optimal The greedy algorithm is insensitive to noise

Stay within a factor of 2 of the super-optimal when the salted error is a factor of 4

The hot spot algorithm performs nearly as well Within a factor of 1.6 – 2 of super-optimal

How to obtain inputs Moving window average for load prediction Using BGP router data to obtain topology information

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Contributions Workload characterization

Study the workload of MSNBC web site

Protocol efficiency Optimize TCP startup

performance for Web transfers

Infrastructure provisioning

Develop placement algorithms for Content Distribution Networks

WorkloadCharacterization

Protocol Efficiency

InfrastructureProvisioning

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Other Work Available at

http://www.cs.cornell.edu/lqiu/papers/papers.html Fast Firewall Implementations for Software and

Hardware-based Routers. Submitted to ACM SIGMETRICS’2001.

Integrating Packet FEC into Adaptive Voice Playout Buffer Algorithms on the Internet. Proceedings of IEEE INFOCOM'2000, Tel-Aviv, Israel, March 2000.

On Individual and Aggregate TCP Performance. 7th International Conference on Network Protocols (ICNP'99), Toronto, Canada, October 1999.