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  1. Outline • Review of Classical Operating Systems - continued • Distributed Systems COP5611

  2. Announcement • The class email list • I created a class email list using ACNS’ service • If you do not receive an email from me this morning, you need to send me an email to add your email address to the list • Materials to make up • In general, the technical issues in distributed operating systems are different to the ones in classical operating systems • For this class, the following undergraduate topics may be helpful • Mutual exclusion (Process synchronization) • Deadlock detection • File systems • Memory management • Scheduling COP5611

  3. Operating System • An operating system is a layer of software on a bare machine that performs two basic functions • Resource management • To manage resources so that they are used in an efficient and fair manner • User friendliness COP5611

  4. Distributed Systems • A distributed system is a collection of independent computers that appears to its users as a single coherent system • Independent computers mean that they do not share memory or clock • The computers communicate with each other by exchanging messages over a communication network COP5611

  5. Distributed Systems – cont. COP5611

  6. Distributed Systems – cont. • Is each of the following systems a distributed system according to our definition? • Workstations at the Computer Science department • World wide web COP5611

  7. Distributed Systems – cont. • Motivations • The availability of powerful microprocessors • Significant advances in communication technology • A group of people working together need to share data and expensive resources COP5611

  8. Distributed Systems – cont. • Advantages • The computing power of a group of cheap workstations can be enormous • Decisive price/performance advantage over traditional time-sharing systems • Resource sharing • Enhanced performance • Improved reliability and availability • Modular expandability COP5611

  9. Distributed System Architecture • The minicomputer model • Consists of several minicomputers, each supports multiple users and provides access to remote resources • The workstation model • Consists of a number of workstations, each supports a single user in general • The processor model • The ratio of the number of processors to the number of users is greater than one COP5611

  10. Distributed System Architecture – cont. • The classification given above does not consider the underlying hardware • Distributed systems are often classified based on the hardware • Multiprocessor systems • Homogenous multi-computer systems • Heterogeneous multi-computer systems COP5611

  11. Basic Distributed Systems 1.6 COP5611

  12. Multiprocessor Systems – cont. • Multiprocessor systems are often divided into two categories • In tightly coupled systems, all processors share the same memory address space and all processors can directly access a global main memory • In loosely coupled systems, not only is the main memory partitioned and attached to processors, but each processor has its own address space • A processor cannot directly access memory attached to other processors COP5611

  13. Multiprocessor Systems – cont. • Based on the vicinity and accessibility of the main memory to the processors, there are three main types of multiprocessor system architectures • UMA – Uniform memory access • NUMA – Non-uniform memory access • NORMA – no remote memory access COP5611

  14. Multiprocessor Systems – cont. • A bus-based multiprocessor system COP5611

  15. Multiprocessors Systems – cont. • A crossbar switch • An omega switching network COP5611

  16. HomogeneousMulticomputer Systems • Compared to multiprocessor systems, building multicomputer systems is relatively easy • Each CPU has a direct connection to its own local memory • The problem is how the CPUs communicate with each other • There are kinds of multi-computer systems • Bus-based / switch-based COP5611

  17. Bus-Based Multicomputer Systems COP5611

  18. Homogeneous Multicomputer Systems – cont. • Grid • Hypercube 1-9 COP5611

  19. Heterogeneous Multicomputer Systems • Most of the distributed systems are built on top of a heterogeneous multicomputer systems • Computers can vary widely • Some of them can be even multiprocessor or homogeneous multicomputer systems • The interconnection network may be highly heterogeneous as well COP5611

  20. Distributed Operating Systems • Hardware for distributed systems is important, but the software largely determines what a distributed system looks like to a user • Distributed operating systems are much like the traditional operating systems • Resource management • User friendliness • The key concept is transparency COP5611

  21. Distributed Operating Systems – cont. • In a truly distributed operating system, the user views the system as a virtual uniprocessor system even though physically it consists of multiple computers • In other words, the use of multiple computers and accessing remote data and resources should be invisible to the user COP5611

  22. Distributed Operating Systems – cont. COP5611

  23. Distributed Operating Systems – cont. • Degree of transparency and performance • There is a trade-off between a high degree of transparency and the performance of the system • Three categories • Distributed Operating System (DOS) • Network Operating System (NOS) • Middleware COP5611

  24. Overview of Different Kinds of Distributed Systems COP5611

  25. Uniprocessor Operating Systems • Separating applications from operating system code through a microkernel. 1.11 COP5611

  26. Multicomputer Operating Systems • General structure of a multicomputer operating system COP5611

  27. Distributed Shared Memory Systems • Pages of address space distributed among four machines • Situation after CPU 1 references page 10 • Situation if page 10 is read only and replication is used COP5611

  28. Network Operating System 1-19 COP5611

  29. Network Operating System – cont. • Two clients and a server in a network operating system. COP5611

  30. Network Operating System – cont. • Different clients may mount the servers in different places. COP5611

  31. Positioning Middleware • General structure of a distributed system as middleware. COP5611

  32. Middleware and Openness • In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications. 1.23 COP5611

  33. The Global Architecture of CORBA COP5611

  34. Object Model in CORBA • The general organization of a CORBA system. COP5611

  35. CORBA Services COP5611

  36. Comparison Between Systems COP5611

  37. Issues in Distributed Operating Systems • Absence of global knowledge • In a distributed system, due to the unavailability of a global memory and a global clock and due to unpredictable message delays, it is practically impossible to for a computer to collect up-to-date information about the global state of the distributed system • Therefore a fundamental problem is to develop efficient techniques to implement a decentralized system wide control • Another problem is how to order all the events COP5611

  38. Issues in Distributed Operating Systems – cont. • Naming • Plays an important role in achieving location transparency • A name service maps a logical name into a physical address by making use of a table lookup, an algorithm, or a combination of both • In distributed systems, the tables may be replicated and stored at many places • Consider naming in a distributed file system COP5611

  39. Issues in Distributed Operating Systems – cont. • Scalability • Systems generally grow with time, especially distributed systems • Scalability requires that the growth should not result in system unavailability or degraded performance • This puts additional constraints on design approaches COP5611

  40. Scalability – cont. • Consider the scalability of centralized design approaches COP5611

  41. Scaling Techniques • The difference between letting: • a server or • a client check forms as they are being filled COP5611

  42. Scalability – cont. An example of dividing the DNS name space into zones. COP5611

  43. Issues in Distributed Operating Systems – cont. • Compatibility • Refers to the interoperability among the resources in a system • Three different levels • Binary level • All processors execute the same binary instruction repertoire • Virtual binary level • Execution level • Same source code can be compiled and executed properly • Protocol level COP5611

  44. Issues in Distributed Operating Systems – cont. • Process synchronization • The synchronization of processes in distributed systems is difficult because of the unavailability of shared memory • It needs to synchronize processes running on different computers when they try to concurrently access a shared resource • This is the mutual exclusion problem as in classical operating systems COP5611

  45. Issues in Distributed Operating Systems – cont. • Resource management • Resource management needs to make both local and remote resources available to uses in an effective manner • Data migration • Distributed file system • Distributed shared memory • Computation migration • Remote procedure call • Distributed scheduling COP5611

  46. Issues in Distributed Operating Systems – cont. • Security and protection • The same two issues as in classical operating systems need to be considered • Authentication • Authorization COP5611

  47. Issues in Distributed Operating Systems – cont. • Structuring • The distributed operating system requires some additional constraints on the structure of the underlying operating system • The collective kernel structure • An operating system is structured as a collection of processes that are largely independent of each other • Object-oriented operating system • The operating system’s services are implemented as objects COP5611

  48. The Client-Server Model • The client-server model provides one organization for a distributed system • The processes are organized into clients and servers • Clients request services from servers which provide services • A server is a process implementing a specific service • A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server’s reply COP5611

  49. Clients and Servers • General interaction between a client and a server. COP5611

  50. An Example Client and Server (1) COP5611