Chapter 15 - Distributed operating systems. This chapter discusses important features of these components and the manner in which these features influence the computation speedup, reliability, and performance that can be achieved in a distributed system.
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Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 1 Copyright © 2008 Distributed Systems • Definition A distributed system consists of two or more nodes, where each node is a computer system with its own memory, some networking hardware, and a capability of performing some of the control functions of an OS – Thus, each node performs some OS functions * Data used by an OS function may be spread across many computers * Non-local data is accessed through the network Chapter 15: Distributed Operating Systems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:2 Copyrightâ2008 Distributed Systems Distributed systems consist of four components – – – – Individual computer systems Network connecting the computer systems Distributed computations Distributed operating system We discuss the basics of these four components in this chapter Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 3 Copyright © 2008 Benefits of distributed systems • Distributed systems provide five key benefits – Resource sharing * Sharing can occur across boundaries of computers – Reliability * High availability can be offered by exploiting multiplicity of resources – Computation speed-up * Processes of an application can be scheduled simultaneously – Communication * Users and processes in different nodes can communicate – Incremental growth * Cost of enhancing a capability is α additional capability desired Made possible by open system standards Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed SlideNo:4 Copyrightâ2008 Nodes in a distributed system Nodes can be of different types – Workstation * Has a single CPU and single user – Minicomputer * Has a single CPU but many users It is also called a process pool node – Cluster * A group of nodes that work together in an integrated manner Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:5 Copyrightâ2008 Architecture of a cluster EachnodemayhavemanyCPUs NodescommunicateamongthemselvesoverhighưspeedprivateLAN WorkingofthenodesisintegratedviaOSsoftware • Provides reliability through Use of RAID and redundancy of CPUs Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 6 Copyright © 2008 Operating systems for a distributed system • The OS must provide – – – – • Resource sharing across boundaries of systems Computation speed-up of applications Reliability Good performance of the distributed system Two kinds of operating systems – Network operating systems * Only provide resource sharing – Distributed operating systems * Integrate functioning of individual computers Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 7 Copyright © 2008 A network operating system • The network OS layer exists between a process and the kernel of OS • It recognizes requests for access to remote resources; implements them • It passes other requests to the kernel Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:8 Copyrightâ2008 Distributed Operating Systems Differences with a conventional OS – A distributed OS integrates functioning of individual computers and scatters processes of an application to various nodes * Achieves computation speed-up and resource efficiency * Helps in providing reliability – Examples * Windows cluster Node manager detects faults, failover manager provides reliability * Sun Cluster software Global process management, distributed file system enable process migration when a failure occurs * Amoeba distributed OS Processes of an application are created in different nodes Chapter 15: Distributed Operating Systems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:9 Copyrightâ2008 Reliable interprocess communication Communication between processes takes place through the network It raises following issues – Naming of processes * Processes should be able to find each other’s network addresses Address of a process is a pair (, ) The domain name service (DNS) is a distributed service for obtaining the IP address of each computer * A name server is a generic name for this arrangement – Reliability of communication * Interprocess messages may be lost due to congestion in the network Chapter15:Distributed OperatingSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:10 Copyrightâ2008 Connection strategies A connection strategy – decides when, and for how long, to set up a connection * A connection is a data path between processes – It is also called a switching technique – It influences communication efficiency and throughput of links – Three connection strategies * Circuit switching Circuit: A connection used exclusively by a pair of processes * Message switching A connection is set up for every message * Packet switching A connection is set up for each packet, which is a standard-size portion of a message Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 29 Copyright © 2008 Connection strategies (a) All messages between the processes use the same connection (b) A connection is set up for each message (c) A connection is set up for each packet in a message Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 30 Copyright © 2008 Routing strategies • The routing function decides which network path would be used by a connection – It enables the system to adapt to changing traffic patterns – Three routing strategies * Fixed routing The same path is used for all communication between a pair of nodes * Virtual circuit A path is selected at the start of a session between processes * Dynamic routing A path is selected when a message or packet is to be sent Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 31 Copyright © 2008 Routing strategies (a) Same path is used for communication between all processes in a pair of nodes (b) A path is chosen for communication between a pair of processes (c) A path is chosen for each message or each packet Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 32 Copyrightâ2008 Network Protocols A network protocol is a set of rules and conventions used to implement communication – It addresses four concerns * Naming of sites * Efficient name resolution * Communication efficiency * Handling of faults – It consists of a hierarchy of protocols that address specific concerns Hence it is called a protocol stack * The ISO protocol consists of protocols * The TCP / IP protocol consists of protocols Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 33 Copyrightâ2008 The ISO protocol The ISO protocol consists of protocol layers – Physical layer * Provides electrical mechanisms for bit transmission – Data link layer * Collects bits into frames, performs error detection and flow control – Network layer * Performs routing and flow control – Transport layer * Assembles packets / splits messages, performs error detection Chapter 15: Distributed OperatingSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:34 Copyrightâ2008 The ISO protocol The ISO protocol (contd) – Session layer * Initiates and terminates sessions between processes * Provides for restart and recovery – Presentation layer * Performs change of data representation, compression, encryption and decryption, etc – Application layer * Provides network interface for applications such as file transfer, e-mail, etc Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 35 Copyright © 2008 Operation of the ISO protocol stack Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 36 Copyright © 2008 The transmission control protocol / internet protocol (TCP / IP) stack • IP is a connectionless, unreliable protocol for communication between hosts • TCP is a connectionoriented reliable protocol • UDP is a connectionless, unreliable protocol Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 37 Copyright © 2008 The TCP / IP protocol stack • Features of the protocols – TCP * Connection-oriented, reliable protocol * Uses a virtual circuit between processes, retransmits on time-out * Employs flow control so that receiver can accept messages at the rate at which the sender sends them It controls retransmission overhead – UDP * Connection-less, unreliable protocol * Used in multi-media applications and video conferencing because loss of packets is tolerable – Higher layer protocols * File transfer, SMTP (e-mail), remote log in, etc Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 38 Copyright © 2008 Network bandwidth and latency • Bandwidth – Rate at which data is transferred over the network * Depends on link capacities, error rates and delays • Latency – Elapsed time between sending and receiving of a byte * Processing time in protocol layers and delays due to network congestion contribute to it * Typically computed for the first of the bytes to be transferred Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 39 Copyright © 2008 Modeling a distributed system • A distributed system is modeled as a graph S = (N, E) where N is the set of nodes E is the set of edges – Two kinds of graph models are used * In a physical model, each node is a computer system and each edge is a communication link * In a logical model, each node is a process and each edge is an interprocess communication channel It is used to model a distributed computation Chapter 15: Distributed OperatingSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:40 Copyrightâ2008 Uses of system models System models are used to determine properties of a system or computation – Impact of faults * Minimum number of faults that would partition the system – Resiliency * k-resiliency: k is the largest number of faults which a system can withstand without disruption in its functioning – Latency between nodes * Minimum latency depends on minimum number of links on a path between two nodes – Cost of sending information to all nodes * Number of messages needed * Used to determine complexity of algorithms Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed SlideNo:41 Copyrightâ2008 Design issues in distributed operating systems Distributed nature of the computing environment raises five significant issues – Transparency of resources and services * Transparency: resource names should not depend on their locations * It simplifies access from different locations – Distribution of control functions * OS functions are performed in many nodes to ensure reliability – System performance * Load balancing is performed to obtain resource efficiency * Special techniques like file caching are used for scalability Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 42 Copyright © 2008 Design issues in distributed operating systems • Design issues (contd) – Reliability * Redundancy of resources is exploited to provide fault tolerance * Special techniques like two phase commit are used – Security * An intruder may corrupt interprocess messages over the network * Special techniques are used for message security and authentication Chapter 15: Distributed Operating Systems Dhamdhere: Operating Systems— A ConceptBased Approach , 2 ed Slide No: 43 Copyright © 2008 ... distributed application may organize data in several ways – Data replication * Copies of data are kept in many nodes of the system It assists in fault tolerance and provides high availability – Data distribution... TheLANisconfinedtoalaboratory,abuildingoraclusterofbuildings TheWANconnectsgeographicallydistantnodes Chapter1 5:Distributed OperatingSystems Dhamdhere: OperatingSystems AConceptưBasedApproach,2ed... Slows down operation of the application – Data migration * Data is moved to the site of the application * May complicate management of replicated data – Process migration * An application process