Distributed Operating System

Transcription

1 Distributed Operating System Mahesh Goyani Assistant Professor, L D College of Engineering Ahmedabad, Gujarat, INDIA mgoyani@gmail.com Web Site: A.Y: Last Updated On:

2 LECTURENOTES ON INTRODUCTIONTO DISTRIBUTEDOPERATINGSYSTEMS 2

3 COURSE DETAIL TEXT BOOKS Title Author Distributed Operating Systems Andrew S. Tanenbaum Publication Prentice Hall 3

4 COURSE DETAIL REFERENCE BOOKS Title Author Distributed Operating Systems Concepts & Design Pradeep K. Sinha Publication PHI Title Author Advanced Concepts in Operating Systems Mukesh Singhal, N.G.Shivaratri Publication Tata McGraw Hill 4

5 COURSE DETAIL PREREQUISITE Basics of Operating Systems CLanguage Basic UNIX Commands Shell Scripting Socket Programming 5

6 INTRODUCTION 6

7 DISTRIBUTED OPERATING SYSTEM ROADMAP ReviewofOSConcepts Introduction to Distributed System Definition& Goal Hardware Concepts Software Concepts Design Issues 7

8 DISTRIBUTED OPERATING SYSTEM MOTIVATION Enhancementinmicroelectronictechnology:8bit16bit32bit64bit Availability of Fast, inexpensive, cost effective computers: Price/performancegain:10 11 Price performance ratio favors use of interconnected, multi processor in place of single, high speed processor Inventionofhighspeedcomputernetwork:LAN&WAN 64 Kbps to 10/100/1000 MBPS networks Centralized Approach Distributed Approach 8

9 DISTRIBUTED OPERATING SYSTEM GOAL Definition: A distributed system is a collection of independent computers that appear to the users of the system as a single computer Two aspects: HARDWARE: Machines are autonomous SOFTWARE:Userthinkofasystemasasinglecomputer Example: E-Commerce business, Bank with large number of branches Each user s workstation + Pool of processors Single file system Acts like classical single processor time sharing system 9

10 GOALS 1. ADVANTAGEOFDS OVERCS Driving force: Economy Grosch slaw: Computing power of a CPU is proportional to the square of its price.(valid for mainframe computers only ) Microprocessor technology has better price/performance ratio 10,000CPU*50MIPS=5,00,000MPIS Single processor need to execute instruction in ns: Impossible Einstein'stheoryofrelativity:Lightcantravel0.6mmin0.002ns Distributed system: Allows many users to work together Parallel system: To achieve maximum speed up on a single problem 10

11 GOALS 1. ADVANTAGEOFDS OVERCS Some applications are inherently distributed(bank, Supermarket chain) Local decisions are made locally Updates query locally whole the day Computer Supported Cooperative Work CSCW: Work together Computer Supported Cooperative Games CSCG: Play together Higher reliability Dominant consideration in critical applications like nuclear plant Incremental Growth Adding or changing mainframe can create havoc 11

12 GOALS 2. ADVANTAGES OF DC OVER INDEPENDENT PCS Why not independent machine? 1. Data sharing Reservation systems 2. Sharing expensive resources like color printer, phototypesetter etc 3. Enhanced person to person communication: Electronic mail A. Faster than paper mail B. Doesnotrequirebothpartytobeonlineatsametimeliketelephone C. Unlike FAX, it is editable 4. Heterogeneous computing environment A. Let jobs run on the most appropriate machine, rather than owner s machine B. Reliable 12

13 GOALS 3. DISADVANTAGES OF DS Problems: 1. Software: A. OS, Programming language, transparency? 2. Communication network A. Lossofmessage B. Saturated network needs replacement 3. Easy way of data sharing A. security Despite of these problems, many people feel that advantages of DS outweigh the disadvantages 13

14 CLASSIFICATION 14

15 HARDWARE CONCEPT FLYNN S CLASSIFICATION EachDSconsistsofmultipleCPUs,whichcanbeorganizedindifferentways Flynn s Classification(1972): 1. SISD: All traditional uniprocessor computers at home and office 2. SIMD: Array processor, Some super computer 3. MISD: No known computer 4. MIMD: All DS 15

16 HARDWARE CONCEPT CLASSIFICATION BY TANENBAUM MIMD Parallel & Distributed Computers Tightly Coupled (Less delay, High Data rate) Loosely Coupled (High delay, Low Data rate) Multi processors (Shared Memory) Parallel system Multi computers (Local Memory) Distributed System Bus Switched Bus Switched Sequent, Encore Ultra computer, RP3 Workstation on LAN Hypercube, Transputer 16

17 HARDWARE CONCEPT 1. BUS BASED MULTIPROCESSOR CPUsareconnectedtocommonbusalongwithmemory Typicalbushas32or64bitaddress,dataandcontrolline BreadssameasAwrites:Coherent Busbecomesbottleneckwithasfewas4-5CPUs SolutionCachememory(Hit+Miss)Memorybecomesincoherent Writethroughcache:Whenwordiswrittentocache,itsupdatedinmemory Snoopy cache: When write occurs in memory, cache entry is either removed or updated with new value in memory(always coherent) Supportupto64processors CPU Cache CPU Cache CPU Cache Memory BUS 17

18 HARDWARE CONCEPT 2. SWITCHED MULTIPROCESSOR To build multiprocessor with more than 64 processors different method is needed Crossbar switch: Divide memory in modules Connect memory module by cross bar switch CPUs can access memory simultaneously No two CPUs can access same memory cell at same time Downside:N M *N C crosspointswitches Omega switch: Fewer switches With n CPUs and n memory units, omega network requires(nlog 2 n)/2switchingstage. Downside: Delay M M M M C C C C Cross point Switch Cross bar Switch C M C M C M C M 2 X 2 Omega Switch 18

19 HARDWARE CONCEPT 2. SWITCHED MULTIPROCESSOR Hierarchical approach: Local + Shared memory NUMA: Non uniform memory access CPU can access its local memory quickly but accessing anybody else s memory is slower NUMA have better average access times than machines based on omega networks Complications: Placement of program and data 19

20 HARDWARE CONCEPT 3. BUS BASED MULTI COMPUTERS Building multi computer without shared memory is easy EachCPUhasdirectaccesstoitsownlocalmemory Problem: How CPUs communicate with each other CPUtoCPUtrafficisseveralordersofmagnitudelowerthanCPUtoMemory Busneednotbehighspeedbackplanebusduetoreducedtraffic Workstation Local Memory CPU Workstation Local Memory CPU Workstation Local Memory CPU Bus 20

21 HARDWARE CONCEPT 4. SWITCHED MULTI COMPUTERS CPUhasdirectaccesstoitsprivatememory Grid:Suitableforproblemthathaveaninherent2Dnature PathlengthgrowsinorderofsquarerootofnumberofCPUs Hypercube:ndimensionalcube(heren=4) Expandittofivedimension,addsetoftwointerconnectedcubestofigure Messagehavetomakeseveralhopstoreachtodestination PathlengthgrowsinlogarithmicorderofnumberofCPUs For n-dimensional hypercube, each CPU is connected to n CPUs 16, 384 CPUs are available nowadays Grid Hypercube 21

22 TYPESOFOPERATINGSYSTEM 22

23 SOFTWARE CONCEPT SOFTWARE CONCEPTS Image of system presented to user is determined by software Unlike hardware, OS can not be distinguished cleanly. Roughly classified as, Loosely coupled software Allows users and machines to be independent of each other Interaction is limited, e.g. communication over LAN If network goes down, independent machine can still continue to work Tightly coupled software Evaluating chess board on multiple processor simultaneously Software for such system require support from both application program andos 23

24 SOFTWARE CONCEPT 1. NETWORK OPERATING SYSTEM Loosely coupled software on loosely coupled hardware Most common combination E.g. Collection of workstations connected by a LAN AllmachinehasitsownOS Allcommandrunslocally,rightontheworkstation Sometimes possible to login in to another workstation by remote login rlogin machineremote Login rcp machine1:file1 machine2:file2remote file copy 24

25 SOFTWARE CONCEPT 1. NETWORK OPERATING SYSTEM This form of communication is extremely primitive Oneapproachistoprovidesharedfilesystem:FileServer File Server maintain file in hierarchical structure Client can import or mount and unmount directories Different client have different view to File System Client and server can run different OS, but should agree on common file format Very limited coordination 25

26 SOFTWARE CONCEPT 2. TRUE DISTRIBUTED SYSTEM NOS are loosely coupled software on loosely coupled hardware Next evolution: Tightly coupled software on same loosely coupled(i.e. multi computer) hardware: Single machine image/ Virtual uniprocessor UsershouldnothaveanyideaaboutexistenceofmultipleCPUsinsystem Characteristics of DS: 1. There must be single, global IPC mechanism, for local and remote communication. 2. Process management(create, destroy, start, stop etc) must be same 3. File system must look same every where(e.g. file name length, visibility) 4. Same system call interface identical kernels run on all CPUs 5. Kernel has control to manage own local resources: Swapping, Paging, Scheduling 26

27 SOFTWARE CONCEPT 3. MULTIPROCESSOR TIMESHARING SYSTEM Tightly coupled software on tightly coupled hardware Special purpose machine exists, like database machine Example: Unix with multiple CPUs: N CPU improve performance N fold Key characteristic: Single RUN QUEUE in shared memory Initialexecutionwouldbeslowduetocachemiss. If possible, allocate the same CPU to the process which comes back after performing I/O To improve performance BusywaitingispreferableincaseofshortI/O Differsfromothersystemsinorganizationoffile:Lockcache Process A Running Cache Process B Running Cache Process C Running Cache Disk E (Ready) D (Ready) C (Running) B (Running) A (Running) Run Queue: D, E OS BUS 27

28 SOFTWARE CONCEPT COMPARISON OF OS Item NOS DOS MOS Does it look like a virtual uniprocessor? NO YES YES Do all have to run same OS? NO YES YES How many copies of OS are there? N N 1 How is communication achieved? Shared files Messages Shared Memory Are agreed upon network protocols required? YES YES NO Is there a single run queue? NO NO YES Does file sharing have well defined semantics? Usually NO YES YES 28

29 DESIGNISSUES 29

30 DESIGN ISSUES 1. TRANSPARENCY BigQuestion:Howtoachievesinglesystemimage? Transparency can be achieved at two different levels: 1. Hide distribution from user: make command in UNIX to compile the program Compilation may proceed in parallel on any machines 2. System call interface can be designed in such a way so that existence of multiple processor is not visible 30

31 DESIGN ISSUES 1. TRANSPARENCY Type Location Migration Replication Concurrency Meaning Users can not tell where software (database, files) and hardware (CPU, printer) resources are located. Machine1:prog.c is not acceptable Resources can move at will withoutchanging their names: Directory hierarchy. /games/news /work/news User cannot tell how many copies exists ( Server forms a ring to serve/forward user request and replicate heavily used files) Multiple users can shareresources automatically (Lock the resource so multiple user can not access simultaneously) Parallelism Activities can happen parallel without user knowing (hardest to achieve). Evaluate chess board on multiple CPU In case of printer, user don t wont transparency 31

32 DESIGN ISSUES 2. FLEXIBILITY Weareinlearningmode,sotheremustbeawaytocorrecterrororbacktrack Designseemreasonablenowmaylaterprovetobewronglater Monolithic Kernel: Today's centralized OS augmented with network facility and integration of remote services System calls are made by trapping the kernel, having the work performed there, and having the kernel return desired result to user process Machineshavetheirowndiskandmanagetheirownlocalfilesystem:UNIX Only advantage is performance Faster than micro kernel(sprite) User User File Server Directory Server Process Server Monolithic Kernel Micro Kernel Micro Kernel Micro Kernel Micro Kernel Includes file, directory and process management Network 32

33 DESIGN ISSUES 2. FLEXIBILITY Micro Kernel: More flexible, provides following services 1. IPC mechanism 2. Some memory management 3. Small amount of low level process management& Scheduling 4. Low level I/O Unlike monolithic kernel, it does not provide file system, directory structure, full process management or much system call handling AllotherOSservicesareimplementedinuserspacelikereadfile,writefile Easy to implement, install and debug services This method is highly modular with well defined interface Userarefreetowritetheirownservice E.g: AMOEBA 33

34 DESIGN ISSUES 3. RELIABILITY One machine goes down, some other machine serve the purpose Aspects of Reliability: 1. Availability: Refers to the fraction of time that the system is usable. Availability can be improved by replicating resources or by enhancing the design such that critical components are not accessed simultaneously Highly reliable system must be highly available. More copies Better availability Chances of inconsistency 2. Security: Resources must be protected 3. Fault tolerance: Proper arrangement of closely connected servers Performance degradation 34

35 DESIGN ISSUES 4. PERFORMANCE Nomeaningofconqueringotherdesignissuesifsystemdoesnotperformwell Running the application on DS should not be worst then running it on single processor system Performance matrices: Responsetime:Timerequiredtofinishasinglejob Throughput: Number of jobs per hour System Utilization: CPU usage Network capacity consumed: Bandwidth used To optimize performance in DS, minimize number of messages Doalltaskonsinglemachine(HardlyappropriateinDS) Fine grained: large number of small computation, high interaction Local Coarse gained: large computation, low interaction, little data Remote 35

36 DESIGN ISSUES 5. SCALABILITY Networkmaygrowwithgrowthofbusiness Expand the network by adding machines Minitel terminal in France Telephone database Terminalsarereadysocanbeextendedtous service,otherdatabaseetc Guiding principle in DS: AVOID CENTRALIZED COMPONENTS Concept Example Centralized component A single mail server for all users Centralized Table A single online telephone book Centralized Algorithm Doing routing based on complete information Potential bottlenecks that should be avoided 36

37 DESIGN ISSUES 5. SCALABILITY Only decentralized algorithm should be used: 1. No machine has complete information about the system state 2. Machine make decisions based only on local information 3. Failure of one machine does not ruin the algorithm 4. There is no implicit assumption that a global clock exists 37

38 AUTHOR S PROFILE Mahesh Goyani has completed his graduation in Computer Engineering from SCET, VNSGU, Surat in 2005 with distinction. He received his Master Degree in field of Computer Engineering with 9.38 CPI (81.03 %) from BVM College, SPU, Anand in He has secured 1 st rank twicein university duringhis master degree. His area of interest is Image Processing, Computer Algorithms and Computer Graphics. He has also done graduation in Gujarati literature from Gujarat University in Publication: He has published many research papers in national and international journals and conferences. He was invited as a SESSION CHAIR in International Conference on Engineering, Science and Information Technology, Tirunelveli, Tamilnadu, Sept He has published three books- two on Computer Graphics and one on Design and Analysis of Algorithms. Editorial: He is the member of technical review committee of International Journal of Computer Science & Issues (IJCSE, Mauritius), Electronics & Telecommunication Research Institute (ETRI, Korea), International journal of Engineering & Technology (IJET, Singapore), International journal of Computer Science & Information Security (IJCSIS, Pittsburg, USA). He has worked as a program committee member and reviewer in many International Conferences and Journals. He is also a life time member of ISTE technical society. Web Site: 38