Class Note for CMPSCI 377 at UMass(59)
Class Note for CMPSCI 377 at UMass(59)
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Date Created: 02/06/15
CMPSCI 377 Operating Systems Fall 2005 Lecture 20 December 01 Lecturer Emery Berger Scribe Avery Harris Today 0 Distributed Systems Page Eviction Network Topologies 0 Network Communication Packets 201 Distributed Systems 2011 General Information 0 A distributed system is a set of physically separate processors connected by one or more communication links 7 contains no shared clock or memory 7 this systems are in broad use today 2012 Parallel vs Distributed Systems 0 Tightlycoupled systems or 77parallel processing77 7 Systems share clocks7 memory7 etc 7 Systems frequent communication 0 Looselycoupled systems or 77 distributed computing77 7 Each systemprocessor has its own memory and runs independent of the OS 7 Systems have infrequent communication 2013 Advantages of Distributed Systems 0 Resource Sharing 7 Resources do not need to be replicated because they are shared 7 Expensive resources can be shared 7 Processors present the same environment to users 201 202 Lecture 20 December 01 0 Computational Speedup 7 n processors7 n times the computational power 7 Problems must be decomposable into subproblems 7 Coordination and communication is required between operating processes 0 Reliability Replication of resource produces fault tolerance This cannot happen with only one machine 7 If one node crashes the user can work on another Systems must be available for all but ve minutes per year Must avoid single point of failure 0 Communication 7 Users7 or processors7 on different systems can communicate Examples of this are mail servers and transaction processing systems 2014 Issues With Distributed Systems 0 Issues Communication Transparency Security Reliability Performance V FP DN E Programming modules 20 2 Networks 2021 General Information 0 Networks should provide efficient7 correct7 and robust message passing between two separate nodes 0 Local area networks are connected in a singled building This networks are extremely fast and reliable 0 Wide area networks are connected across a large geographical area 2022 Network Topologies 0 Connection of the nodes impact the fault tolerance and cost of a network 0 There are two basic topologies l PointtoPoint 2 Bus Lecture 20 December 01 203 2023 Pointto Point Networks 0 Fully Connected 7 This is not used in common day practice 7 Each message takes one hop 7 Node failure will not bring this network down but it is very expensive 0 Partially Connected 7 Links between some but not all nodes 7 Sending messages requires several hops 7 This setup is not ideal because one node failure can result in the entire network going down 0 Tree Structure 7 Network is connected in a tree structure This is where every node is connected through a child or parent node 7 Messages are fast between descendants They are of order logn timer 7 Network is vulnerable to failure especially at nodes located at a higher position within the tree 0 Star Network 7 All nodes are connected to a central nodei Each message takes two hops Central node is critical need to take action to make that node very reliable Not fault tolerant 0 One directional Ring 7 Nodes are connected in the same fashion as a token ringi With max hops being nli 7 This system is very inexpensive but it only takes one node failure to bring down the entire network 0 Bi directional Ring 7 Nodes are connected like a onedirection ring but with nodes being able to communicate forwards and backwards This network gives n2 max hops 7 This network is inexpensive but is still not fault tolerant as it would only take two node failures to take down this network 0 Doubleconnected Ring 7 Nodes are connected as in the bi directional ring plus one additional nodei This network give n4 max hops 7 This network is more expensive but it is fault tolerant as it would take four node failures to take down this network 204 Lecture 20 December 01 2024 Bus Network Topology 0 Bus nodes are connected to a common network While this is very fast and reliable7 it is not commonly utilized because of the poor scalability 7 Linear Bus 96 Single shared link Nodes connect directly to each other via bus 96 Network is inexpensive and fault tolerant 7 Ring Bus 96 Single shared circular link 96 Same pros and cons as Linear Bus 2025 Principles of Network Communication 0 Data is broken down into packets The packets are then sent through networks These protocols are based on agreed communication standards 2026 Network Layers 0 Application Data is used by applications Presentation Data format conversion Session lmplement communication strategy 0 Transport Reliable endto end communication Network Routing and congestion control Data Link Control Reliable pointto point communication over reliable channel 0 Physical Electrical signaling across wire 2027 TCPIP Protocal Stack o TCP Reliable protocol that ensures that packets are received in order 0 UDP Unreliable protocol that is fast7 but does not ensure that packets are received in order 2028 Packets 0 Contain all information needed to recreate original message 0 Packets may arrive out of order but can be put back in order through the sequence numbers of the packets 0 Data segment contains header for higher protocol layers
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