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Load Balancing in Distributed Systems. Nalini Venkatasubramanian nalini@ics.uci.edu. Motivation. A given computer is overloaded Must decrease load and maintain certain characteristics Scalability Performance Throughput Ideally, this should be done transparently Solution: Load Balancing.

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load balancing in distributed systems

Load Balancing in Distributed Systems

Nalini Venkatasubramanian

nalini@ics.uci.edu

motivation
Motivation
  • A given computer is overloaded
    • Must decrease load and maintain certain characteristics
      • Scalability
      • Performance
        • Throughput
    • Ideally, this should be done transparently
    • Solution: Load Balancing

Global Distributed Systems and Multimedia

introduction to load balancing
Introduction to Load Balancing
  • Distributed resource allocation
    • Can be thought of as “distributed scheduling”
      • Deals with distribution of processes among processors connected by a network
      • Handles issues such as deciding which process should be handled by a given processor
    • Can also be influenced by “distributed placement”
      • Important in data intensive environments and applications
      • Data placement may force process placement

Global Distributed Systems and Multimedia

load balancing relationships
Load Balancing Relationships
  • Load Balancer
    • Manages resources
    • Resource assignment depends on policy or policies in effect
  • Client
    • Requests resources
    • Requests services

Global Distributed Systems and Multimedia

load balancing overhead
Load Balancing Overhead
  • Satisfy client resource access without imposing large amounts of overhead
    • Performance
      • How well resources are managed
    • Efficiency
      • Cost of accessing and using a resource obtained through a load balancer

Global Distributed Systems and Multimedia

load balancing issues
Load Balancing Issues
  • When to migrate processes or forward requests
  • Which processor should be chosen to handle a given process or request
  • Should processes be moved off a computer
  • How should searching for lightly loaded computer be performed

Global Distributed Systems and Multimedia

load balancing issues cont
Load Balancing Issues (cont.)
  • When should load balancing decisions be made
  • What should be taken into account when making the above decisions
  • How should old data be handled
  • Should load balancing data be stored centrally, or in a distributed manner

Global Distributed Systems and Multimedia

load balancing issues cont1
Load Balancing Issues (cont.)
  • Should computers make decisions together
  • What is the performance/overhead tradeoff incurred by load balancing
  • Prevention of overloading a lightly loaded computer

Global Distributed Systems and Multimedia

load balancing techniques
Load Balancing Techniques
  • Basically two ways to perform load balancing
    • Statically
      • Resource is allocated once
    • Dynamically
      • Resource is allocated and managed (possibly dynamically reallocated) to ensure balanced load

Global Distributed Systems and Multimedia

static load balancing
Static Load Balancing
  • Resource allocation is performed once
    • Once resource is allocated it remains allocated (for what duration??)
  • Scheduling decisions are made
    • Deterministically
    • Probabilistically

Global Distributed Systems and Multimedia

static load balancing cont d
Static Load Balancing – cont’d
  • Advantages
    • State generally need not be stored
      • Simplifies implementation
    • Less network traffic due to load balancing related messages
  • Disadvantages
    • Poor resource utilization
      • A given resource may be used much more than others
    • Does not adjust to fluctuations in the load
      • Possible for resource to become overloaded

Global Distributed Systems and Multimedia

static load balancing cont d1
Static Load Balancing – cont’d
  • Example of Static Load Balancing
    • Forwarding processes/requests to a given computer based on dynamically assigned addresses (e.g. via DNS)
      • Web servers (e.g. CNN)
      • UCI host ea.uci.edu is load balanced

Global Distributed Systems and Multimedia

dynamic load balancing
Dynamic Load Balancing
  • Attempts to maintain a balanced load by managing resources while a resource is in use
    • May involve the following
      • Process migration
      • Disabling further access to a resource until a later time
      • Adding new resources “on-the-fly”

Global Distributed Systems and Multimedia

dynamic load balancing strategies
Dynamic Load Balancing Strategies
  • Distributed versus non-distributed
    • Should load information be stored centrally or across several hosts
      • Simplicity versus overhead and reliability
  • Cooperative versus non-cooperative
    • Should decisions be made by a single load balancer or several
      • Globally managed resources versus locally managed resources

Global Distributed Systems and Multimedia

dynamic load balancing strategies cont d
Dynamic Load Balancing Strategies - cont’d
  • Adaptive versus non-adaptive
    • Should previous data effect scheduling decisions
  • Preemptive versus non-preemptive
    • Should a running process be preempted in favor of another process, or for migration to another resource

Global Distributed Systems and Multimedia

case studies
Case Studies
  • Load Balancing for Web Servers
  • Load Balancing for Parallel Computers
  • Load Balancing for Multimedia Applications

Global Distributed Systems and Multimedia

multimedia applications
Multimedia Applications

Electronic

Commerce

Video

Servers

Global

Entertainment

Network

Web Servers

Distance Learning

Graphics Processing

Global Distributed Systems and Multimedia

Tele-medicine

Requirements - Availability, Reliability, Quality-of-Service, Cost-effectiveness, Security

multimedia load management
Multimedia Load Management
  • Primary focus
    • resource optimization across streams and resource management across servers.
      • Quality of Service
        • continuous delivery requirement
        • minor violations of performance requirements
      • Admission Control - resource reservation/negotiation
      • Media Delivery - Resource scheduling (CPU,Disk)
    • Resource Mgmt. Implies Admission Control
      • Caching, VCR Control, Server Selection, Data Placement

Global Distributed Systems and Multimedia

distributed mm servers
Distributed MM Servers
  • Video Server Topology
      • Partitioned Server
      • Externally Switched
      • Fully Switched
  • Video File Placement
    • Heterogeneous Workload - large/small, hot/cold
    • Online Placement
    • Good placement is important
      • dynamic replication time-consuming, dynamic load-bal complementary

Global Distributed Systems and Multimedia

dynamic load balancing1
Dynamic Load Balancing
  • Adapts to statistical fluctuations and changing access patterns
  • Dynamic Migration
    • Deals with poor initial placement
  • Replication
    • Dynamic Segment Replication
      • partial replication (quick response, less expensive)
    • Total Replication
      • on-demand vs. predictive

Global Distributed Systems and Multimedia

load management of distributed mm servers
Load Management of Distributed MM Servers
  • Adaptive Scheduling
      • Assigns requests to servers based on demand and load factors.
      • Invokes replication-on-demand, request migration
  • Predictive Placement
      • Invokes dereplication
  • Optimizations
      • Eager Replication
      • Lazy Dereplication

Global Distributed Systems and Multimedia

a scalable video server architecture
A Scalable Video Server Architecture

Distribution Network

requests

data

Distribution

Controller

Data

Source

Data

Source

Data

Source

Tertiery

Storage

...

control

Global Distributed Systems and Multimedia

architectural view of a networked mm system
Architectural View of a Networked MM System

Qos Broker and Load Management System

Node

Manager

Node

Manager

Node

Manager

Node

Manager

...

Local Data Streaming

Global Distributed Systems and Multimedia

resources in a video server
Resources in a Video Server

Client

Client

Network

Processing

Module

Communication

Modules

Data Manipulation

Modules

Storage

Modules

Global Distributed Systems and Multimedia

load placement scenario
Load Placement Scenario

Data

Source

S2

Data

Source

S1

Storage:

2 objects

Bandwidth:

8 requests

Storage:

8 objects

Bandwidth:

3 requests

Access Network

...

Clients

Global Distributed Systems and Multimedia

characterizing server resource usage
Characterizing Server Resource Usage
  • Ability to service a request on a server depends on:
    • resource available
    • characteristics of a request
  • Load factor(LF) for a request:
    • represents how far a server is from request admission threshold.

LF (Ri, Sj) = max (Dbi/DBj , Mi/Mj , CPUi/CPUj , Xi/Xj)

Global Distributed Systems and Multimedia

adaptive scheduling
Adaptive Scheduling
  • When the broker receives a request Ri for a video object Vi :
      • Consider only data sources that have a copy of Vi.
      • Consider only data sources tha have sufficient resources to support Ri.
      • Chooser server for which LF (Ri, Sj) is a minimum.
      • If no such server exists
        • Reject request.
        • Perform replication-on-demand.
        • Perform request migration.

Global Distributed Systems and Multimedia

predictive data placement
Predictive Data Placement
  • Determines when, where and how many replicas of a video object.
  • Initiated periodically.
  • Results in an assignment of replicas to data sources.
  • Greedy algorithm that uses revenue generated as a metric.

Global Distributed Systems and Multimedia

the greedy cost placement matrix
The Greedy Cost Placement Matrix

PM(Vi, Sj) is the maximum revenue that can accrue from allocating Vi to Sj.

Greedy heuristic: Map(Vi,Sj) = 1 if PM(Vi,Sj) = a b max(PM(Va,Sb))

Global Distributed Systems and Multimedia

optimizations
Optimizations
  • To minimize the overhead of replication
    • Eager replication
      • Replication of video object in anticipation
      • Performed when server resources are free
    • Lazy Dereplication
      • Critical nature of storage resources
      • Mark reusable resources, reclaim disk space later
      • If disk blocks are not overwritten, can be reclaimed

Global Distributed Systems and Multimedia

life of a video object
Life of a video object

Global Distributed Systems and Multimedia

performance evaluation policies
Performance Evaluation Policies

Global Distributed Systems and Multimedia

performance evaluation startup latencies
Performance Evaluation - Startup Latencies

Global Distributed Systems and Multimedia

performance of the basic configuration
Performance of the basic configuration

p1

p2

p3

p4

Global Distributed Systems and Multimedia

performance evaluation varying replication bw
Performance Evaluation - Varying Replication BW

Global Distributed Systems and Multimedia

performance evaluation summary
Performance Evaluation Summary
  • P1: entails high startup latency, requires high storage and replication bandwidth.
  • P2: Unacceptably poor performance.
  • P3: Similar performance to P4 in many cases.
      • At low transfer bandwidths, P4 outperforms P3.
  • P4: Performs well in all cases.

Global Distributed Systems and Multimedia