A brief overview about

Distributed Systems

Group A4

Chris SunBryan MadenMin Fang

Why we need distributed system?

Computers become cheaper

More computers exist

Share resources

Connected together

Centralized v.s. Distributed systems

Mainframes Computerclusters

Characteristics

A collection of independent computers connected through networks.

Characteristics

Providing a single coherent system to users.

hiding the difference between computers and the communications.

Characteristics

Easy to expand and scale.

Replacing, add, and deletecomponents

Characteristics

Middleware

An additional layer of software to hide the heterogeneity of underlying platforms from applications

Middleware

Characteristics

Data flow control is more important than computing.

I/O bound task

CharacteristicsNot proper for tasks of large computing.

A single computer is more efficient for computing.

Problem division and solution combination

Characteristics

LAN or WAN

Goals

Connecting

Transparency

Open

Scalable

Connecting

Easy to share resource

Economics

Transparency

Hide difference in data representation and how a

resource is accessed

Access

Transparency

Hide where a resource is located

Location

Logic Name

Transparency

Hide that a resource may move to another location

Migration

Transparency

Hide that a resource may be moved to another location while in use

Relocation

Transparency

Hide that a resource is replicated

Replication

Transparency

Hide that a resource may be shared by several competitive users

Concurrency

Locking

Transparency

Hide the failure and recovery of a resource

Failure

Transparency

Hide whether a (software) resource is in

memory or on disk

Persistence

Open

Offer services according to standard rules that describe the system and

semantics of those services

Protocol and IDL

Open

Flexible

Separating policy and mechanism

Rich set of parameters

Increase complexity

Scalable

Size

Geography

Administration

Scalable

Decentralization

Data Services Algorithms

Scalable

Techniques

Async Distributed Replicate

DNS zones Cache

Hardware types

Multiprocessor

A single physical address space shared by all CPUs

Multiprocessor

Bus

Coherent

Overloaded

Multiprocessor

Cache

90% hit rate

512K~1M

Multiprocessor

Crossbar

N2

Multiprocessor

Omega

N/2log2 N

Multicomputer

Each machine has its own private memory

MulticomputerHomogeneous

OS Network Memory

Parallel

Multicomputer

Bus

Ethernet

25~100 nodes

Multicomputer

Switch

Mesh Hypercube

Multicomputer

Hypercube

Dimension = N

# Node = 2N

# Link = k2k-1

Multicomputer

Physical routing distance

Hamming distance

Types of Architectures

• Bus • Fully Connected• Ring• 2D

– Grid

– Torus

• HyperCube• Fully connected

2D - Grid

• Grid or mesh– Matrix– Switches are connected to many other switches

but only one processor.

SwitchProcessor

2D - Torus

• Each column is a ring

• Each row is a ring.

• More connections – more communication.

SwitchProcessor

Bandwidth

• Bisection Bandwidth– Separate network into two halves.

– Sum the bandwidth of the lines crossing the imaginary dividing line.

• Examples:– Bus has Bisection Bandwidth of one since only one con

nection between nodes.

– Grid = 4

– Torus = 8

Fault Tolerance

• The bigger the bandwidth the better the fault tolerance.

• Bus network has low fault tolerance.– When the bus is down, the communication is

down.

• Fully connected network has highest fault tolerance.– Also most expensive. (messaging, hardware)

Leads to …

• Grid networks are starting to be developed to share processors and act as a single unit.

• Applications in Gaming, workplace.– Users have more computing power at less cost. – Users in one time zone can use grid in other

time zone where those users are asleep. Continual use of computing power.

References

• Computer Architecture – A Quantitative Approach. Second Edition, Hennessy & Patterson, Morgan Kaufmann, 1996.

The End