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Web Server Load Balancing/Scheduling. Asima Silva Tim Sutherland. Outline. Web Server Introduction Information Management Basics Load Sharing Policies FLEX WARD EquiLoad AdaptLoad Summary Conclusions Future Work. Request enters a router

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web server load balancing scheduling

Web Server Load Balancing/Scheduling

Asima Silva

Tim Sutherland

outline
Outline
  • Web Server Introduction
  • Information Management Basics
  • Load Sharing Policies
    • FLEX
    • WARD
    • EquiLoad
    • AdaptLoad
  • Summary
  • Conclusions
  • Future Work
introduction to web server load balancing
Request enters a router

Load balancing server determines which web server should serve the request

Sends the request to the appropriate web server

Request

Response

Web Servers

Introduction to Web Server Load Balancing

Internet

Router

Load-Balancing Server

Traditional Web Cluster

issues
Issues
  • Efficiently processing requests with optimizations for load balancing
    • Send and process requests to a web server that has files in cache
    • Send and process requests to a web server with the least amount of requests
    • Send and process requests to a web server determined by the size of the request
slide8
FLEX
  • Locality aware load-balancing strategy based on two factors:
    • Accessed files, memory requirements
    • Access rates (working set), load requirements
  • Partitions all servers into equally balanced groups
  • Each server transfers the response to the browser to reduce bottleneck through the router (TCP Handoff)
flex diagram
Flex Diagram

S1

S2

Requests

Forwards

Request

S3

S4

To Client Browser

S5

S6

W(S1) ≈ W(S2) ≈ W(S3) ≈ … ≈ W(S6)

Ar(S1) ≈ Ar(S2) ≈ Ar(S3) ≈ … ≈ Ar(S6)

flex cont
FLEX Cont.
  • Advantages:
    • Highly scalable
    • Reduces bottleneck by the load balancer
    • No software is required
    • Reduces number of cache misses
flex cont ii
FLEX Cont. II
  • Disadvantages:
    • Not dynamic, routing tale must be recreated
    • Only compared to RR
    • Number of access logs required on each server could be tremendous
    • Responsibility of load-balancing and transferring response is given to web servers – unorganized responsibility
    • How often to update access rates and working sets? Monitor?
slide12
WARD
  • Workload-Aware Request Distribution Strategy
  • Server core are essential files that represent majority of expected requests
  • Server core is replicated at every server
  • Ward-analysis computes the nearly optimal core size determined by workload access patterns
    • Number of nodes
    • Node RAM
    • TCP handoff overhead
    • Disk access overhead
ward cont

Distributor

Server

Server

Dispatcher

Switch

Distributor

Server

Dispatcher

Front End

Front End

Distributor

Server

Server

LAN

LAN

WARD Cont.
  • Three components: dispatcher (load balancer), distributor (router), web server
  • Three progressive architectures:

WARD

CARD

LARD

Dispatcher

Distributor

Server

Switch

Front End

Dispatcher

Distributor

Server

LAN

Single front-end distributor, centralized dispatcher

Co-located distributor and server

Co-located distributor, server, and dispatcher

ward diagram
WARD Diagram

S1

S2

S3

Queue:

Queue:

Requests

Queue:

S4

Queue:

S5

S6

  • Each computer is a distributor
  • and a dispatcher

Queue:

Queue:

ward cont ii
WARD Cont. II
  • Similar to FLEX, sends response directly to client
  • Minimizes forwarding overhead from handoffs for the most frequent files
  • Optimizes the overall cluster RAM usage
  • “by mapping a small set of most frequent files to be served by multiple number of nodes, we can improve both locality of accesses and the cluster performance significantly”
ward cont iii
WARD Cont. III
  • Advantages:
    • No decision making, core files are replicated on every server
    • Minimizes transfer of requests and disk reads, both are “equally bad”
    • Outperforms Round Robin
    • Efficient use of RAM
    • Performance gain with increased number of nodes
ward cont iv
WARD Cont. IV
  • Disadvantages:
    • Core files are created on past day’s data, could decrease performance up to 15%
    • Distributed dispatcher increases the number of TCP requests transfers
    • If core files not selected correctly, higher cache miss rate and increased disk accesses
equiload
EquiLoad
  • Determines which server will process a request determined by the size of the requested file
  • Splits the content on each server by file size, forcing the queues sizes to be consistent.
equiload solves queue length problems
EquiLoad Solves Queue Length Problems
  • This is bad

Queue

Queue

  • This is better

Queue

Queue

equiload diagram
EquiLoad Diagram

S1

S2

1k-2k

Requests

2k-3k

Forwards

Request

S3

S4

3k-10k

10k-20k

To Client Browser

S5

S6

20k-100k

>100k

equiload22
EquiLoad
  • Advantages
    • Dynamic repartitioning
    • Can be implemented at various levels
      • DNS
      • Dispatcher
      • Server
    • Minimum queue buildup
    • Performs well under variable workload and high system load
equiload23
EquiLoad
  • Disadvantages
    • Cache affinity is neglected
    • Requires a front end dispatcher
    • Distributor must communicate with servers
    • Thresholds of parameter adjustment
equiload adaptload
EquiLoad  AdaptLoad
  • AdaptLoad improves upon EquiLoad using “fuzzy boundaries”
    • Allows for multiple servers to process a request
    • Behaves better in situations where server partitions are very close in size
adaptload diagram
AdaptLoad Diagram

S1

S2

1k-3k

Requests

2k-4k

Forwards

Request

S3

S4

3k-10k

8k-20k

To Client Browser

S5

S6

15k-100k

>80k

summary
Summary

FLEX

EquiLoad, AdaptLoad

WARD

conclusions
Conclusions
  • There is no “best” way to distribute content among servers.
  • There is no optimal policy for all website applications.
  • Certain strategies are geared towards a particular website application.
future work
Future Work
  • Compare and contrast the three policies
  • Figure out how often nodes should be repartitioned
  • Compare each policy to a standard benchmark
  • Figure out which policy works in a particular environment
questions
Questions?
  • Anyone have one?