This document discusses distributed computing fundamentals and distributed operating system design. It covers motivations for distributed computing like availability of inexpensive processors and networks. It describes types of interconnected multiprocessors like tightly and loosely coupled systems. It outlines different distributed system models including minicomputer, workstation, workstation-server, processor pool, and hybrid models. It discusses advantages of distributed systems like applications being inherently distributable, information and resource sharing, better price-performance, higher reliability, and flexibility. It defines what a distributed operating system is and covers challenges in distributed OS design like lack of a global state and asynchronous message passing.

1. CS 704DAdvanced Operating System(Distributed Computing)Debasis Das

2. Distributed Computing FundamentalsMIT CS704D Advanced OS Class of 20112

3. Motivations for Distributed ComputingFast but inexpensive processors are available aplentyAvailability of cost effective and efficient communication networks Result is ; distributing the computing load even over large geographical distances is an cost effective option now.MIT CS704D Advanced OS Class of 20113

4. Types of Interconnected MultiprocessorsTightly coupled systemsProcessors in the system operate through a common address space(a common main memory)Loosely couple systemEach processor has its own private memory and are on different address spacesMIT CS704D Advanced OS Class of 20114

5. Block DiagramsMIT CS704D Advanced OS Class of 20115Tightly coupledCPUCPUCPUCPUSystemwideShared memoryLocal MemLocal MemLocal MemLocal MemCPUCPUCPUCPUCommunication NetworkLoosely coupled

6. A Distributed Computing SystemA collection of processorsInterconnected by a networkEach processor has its own local memory & peripheralsCommunication between processors is by messagesA node is a local processor with its own local memory and peripheralsAll others are “remote” nodesMIT CS704D Advanced OS Class of 20116

7. Distributed System ModelsMinicomputer ModelWorkstation ModelWorkstation-server modelProcessor pool ModelHybrid modelMIT CS704D Advanced OS Class of 20117

8. Minicomputer ModelMIT CS704D Advanced OS Class of 20118CommunicationnetworkMinicomputerMinicomputerMinicomputerMinicomputer

9. Mini-computer ModelA user is connected to his local machineCan access to the remote nodesResource sharingMIT CS704D Advanced OS Class of 20119

10. Workstation ModelMIT CS704D Advanced OS Class of 201110WorkstationWSWSComunicationnetworkWSWSWS

11. Workstation ModelUser submits work to the local workstationIf the local station is not able to manage the processing, it will find one idle machines and transfer processes to other workstationsProblems are How to find an idle WSHow to transfer a process & execute itWhat happens to the remote process if user logs on to an WS that was idle until now and was given for a remote process executionMIT CS704D Advanced OS Class of 201111

12. Workstation-Server modelMIT CS704D Advanced OS Class of 201112WSWSWSWSWSWSCommunicationnetworkServerServerServer

13. Workstation-Server modelAdvantages compared to WS modelCheaper to provides servers with the required services than providing it locally. e.g file servicesAny WS can be used to access the file serversWith diskless workstations, it is easier to maintain a few large disks centrallyProcess migration is not necessary, request response protocol (client server model of communication)User gets guaranteed response time as workstations are not used for remote processes, processing power may be wastedMIT CS704D Advanced OS Class of 201113

14. Processor Pool ModelMIT CS704D Advanced OS Class of 201114TerminalsNetRunServerFile Server

15. Processor Pool ModelCompared to WS-Server modelProvides better resource utilizationAdditional services can be provided without the need for more processorsNot very suitable for high performance interactive ( such as graphics & windowing) applications, WS-server models probably will work betterMIT CS704D Advanced OS Class of 201115

16. Hybrid ModelReal models depend of the workload being handled.The work station server is a model suitable model for users doing most work locally, doing a little bit of interactive work, send emails, execute simple programs etc.Workstation-server configurations may have pools of processors. These pools can be used for jobs that are compute intensiveMIT CS704D Advanced OS Class of 201116

17. Advantages of Distributed Computing SystemsDesign of distributed systems are more complex than centralized ones. That’s because communication and security issues have to be handled additionally. Still the advantages are Applications are inherently distributable in many casesInformation sharingResource sharingBetter price performanceShorter response times, higher throughputsHigher reliabilityExtensibility & incremental growthFlexibility in meeting user needsMIT CS704D Advanced OS Class of 201117

18. Inherently Distributed ApplicationsApplications that need local processing as well as global level processing and viewing requirementsEmployee database of a multi branch companyBankingAirlines ticketingMIT CS704D Advanced OS Class of 201118

19. Information Sharing among Distributed UsersPerson to person communication over distance. In a distributed system work at a local node can be easily shared at remote nodes tooThus users distributed at diverse areas can collaborateGroupware is possible and is a natural for distributed computingMIT CS704D Advanced OS Class of 201119

20. Resource SharingBesides informationSoftware in librariesDatabasesHardware resources can be sharedMIT CS704D Advanced OS Class of 201120

21. Better Price PerformanceIncreasing performance yet falling prices of processorsBetter & higher speed networks being available at higher reliability yet lower pricesBetter resource sharingMIT CS704D Advanced OS Class of 201121

22. Shorter Response & higher ThroughputIf the problem can be split suitablyMore processors will reduce processing timeThroughput will increaseBetter resource utilizationMIT CS704D Advanced OS Class of 201122

23. Higher ReliabilityRedundant drives, back up copies etc. can be easily arranged in a distributed systemAvailability is better Fail soft is possible etc.MIT CS704D Advanced OS Class of 201123

24. Extensibility & Incremental GrowthAdditional resources can be added incrementallyAdditional processors can be added when neededAddition of resources does not disrupt the work too muchMIT CS704D Advanced OS Class of 201124

25. Flexibility in Meeting User NeedsOrdinary data processingHigh performance computingWork can be distributed to specialized processing unitFor example, interactive jobs can run on local work stations while processor pools can manage compute intensive jobsMIT CS704D Advanced OS Class of 201125

26. What is a Distributed OS?“ An operating System of is a program that controls the resources of a computer system and provides its users with an interface or virtual machine that is more convenient to use than the bare machine.”- Tannenbaum & Van Renesse [1985]Thus what OS needs to do arePresent users with a VM that is easier to program than the underlying hardwareManage resources, keep track of resource users, track requests, account usage, be the umpire in deciding who gets whatMIT CS704D Advanced OS Class of 201126

27. NOS vs. DOSSystem ImageIn case of NOS it is known to the users that distinct machines working cooperatively through a communication system. Whereas a DOS provides a single machine imageAutonomySystems are autonomous. In DOS the operating system manages all the machinesFault tolerance capabilityNo fault tolerance, DOS provides high levels of toleranceMIT CS704D Advanced OS Class of 201127

28. Definition of a DOSAgain by Tannenbaum & Van RenesseA distributed OS is one that looks to its users like an ordinary centralized operating system but runs on multiple, independent central processing units. The key concept here is transparency. In other words, the use of multiple processors should be invisible(transparent) to the user. Another way of expressing the same idea is to say that the user views the system as a “virtual uniprocessor”, not as a collection of distinct machines.MIT CS704D Advanced OS Class of 201128

29. DifficultiesDo manage resources optimally, the manager must be aware of the accurate status of every resource. However, a distributed system never has thatResources are physically separatedNo common clockDelivery of message may be delayed or even lostNo accurate status information is thus available, yet the OS has to try and run things optimallyMIT CS704D Advanced OS Class of 201129

30. DOS Design IssuesMIT CS704D Advanced OS Class of 201130

31. Features of a DOSTransparencyReliabilityFlexibilityPerformanceScalabilityHeterogeneitySecurityEmulation of existing OSMIT CS704D Advanced OS Class of 201131

32. TransparencyAccess transparencyUniform access whether remote or local; uniform system calls and naming conventionsLocation transparencyName transparency & user mobilityReplication transparencyNaming & replication controlFailure transparencyRedundancy; servers, communication etc.Migration transparencyAutomatic, no name change, message passing (receiver has migrated)Concurrency transparencyEvent driven ordering, mutual exclusion, no-starvationPerformance transparencyAuto reconfig, scaling should not cause disruptionMIT CS704D Advanced OS Class of 201132

33. ReliabilityFaults can be fail-stop or Byzantine; prevention includesFault AvoidanceGood design practices, low failure components, redundancy etc.Fault ToleranceRedundancy, distributed controlFault Detection & RecoveryAtomic transactions, stateless servers, ack & time out based messagesMIT CS704D Advanced OS Class of 201133

34. FlexibilityEase of modificationsWhen required it should be easy to modify sub-systemsEase of enhancementWhen required it should be easy to enhance featuresStructure of the kernelMonolithic kernel or a microkernelModular, server based services, easy to implement and modifyThere is a small performance penaltyMIT CS704D Advanced OS Class of 201134

35. PerformanceBatch, if possibleCache whenever possibleMinimize copying of dataMinimize network trafficTake advantage of fine grained parallelism for multiprocessingMIT CS704D Advanced OS Class of 201135

36. ScalabilityAvoid centralized entitiesAvoid centralized algorithmsPerform most operations on client workstationsMIT CS704D Advanced OS Class of 201136

37. HeterogeneityFormat related issues could be a big hurdleFormat interchange conversions will have to be supportedBetter to have an intermediate internal formatMIT CS704D Advanced OS Class of 201137

38. SecuritySender needs to know the right receiver received the messageThe receiver needs to have assurance that a genuine sender sent the messageBoth sender and receiver need to be assured that the contents of the message was not tampered with during transmissionMIT CS704D Advanced OS Class of 201138

39. Emulation of Existing Operating SystemNew developments will use the API of the new OSNecessary for the ability to run existing applicationsBackward compatibilityMIT CS704D Advanced OS Class of 201139