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This introduction provides an overview of distributed systems, including their definition, advantages over centralized systems, and different categories of multiple CPU computer systems. It also covers tightly-coupled and loosely-coupled architectures, multiprocessors, and multicomputers.
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1. Introduction • Two advances in computer technology: • A. The development of powerful microprocessors. • B. The invention of high-speed computer networks.
The advances make it possible to put together computing systems composed of large numbers of CPUs connected by a high-speed network. • They are called distributed systems.
What is a distributed system? • A distributed system is a collection of independent computers that appear to the users of the system as a single computer. • Eg. 1 a network of workstations in a university or company department. • Eg. 2 a factory full of robots, each containing a powerful computer for handling vision, planning, communication, and other tasks. • Eg. 3 a large bank with hundreds of branch offices all over the world.
What is a centralized system? • A centralized system (or a single-processor system) consists of a single CPU, its memory, peripherals, and some terminals.
Multiple CPU computer system categories • SISD: a computer with a single instruction stream and a single data stream. e.g. all traditional uniprocessor computers (those having only one CPU), from personal computers to large mainframes. • SIMD: single instruction, multiple data stream. e.g. array processors with one instruction unit that fetches an instruction, and then commands many data units to carry it out in parallel, each with its own data.
MISD: multiple instruction stream, single data stream. e.g. no known computers fit this model. • MIMD: multiple instruction stream, multiple data stream. e.g. a group of independent computers, each with its own program counter, program, and data.
Hardware MIMD Parallel and distributed computers Tightly coupled Loosely coupled Multiprocessors (shared memory) Multicomputers (private memory) Switched Switched Bus Bus Sequent, Encore Hypercube, Transputer Ultracomputer, RP3 Workstations on a LAN
Tightly-coupled system: the delay experienced when a message is sent from one computer to another is short, the data rate is high. • Loosely-coupled system: the delay is large and the data rate is low.
Bus-Based Multiprocessors CPU CPU CPU Memory Cache Cache Cache Bus
Drawback • As few as 4 or 5 CPUs, the bus will be overloaded. • Solution: cache memory. • Problem with cache: incoherent. • Solution to incoherent: snoopy write through cache.
Write through: whenever a word is written to the cache, it is written through to memory as well. • Snoopy cache: a cache is always “snooping” on the bus. Whenever it sees a write occurring to a memory address present in its cache, it either invalids the entry or updates it.
Switched Multiprocessors-crossbar switch Memories M M M M CPUs C Crosspoint switch C C C
Cross switch • Advantage: many CPUs can be accessing memory at the same time. • Disadvantage: need N2 crosspoint switches with n CPUs and n memories. • One solution: use omega network.
Switched Multiprocessors-omega switch 2 X 2 switch C M C M M C M C An omega switching network
Omega network • With n CPUs and n memories, it requires log2n switching stages, each containing n/2 switches, for a total of (n log2n)/2 switches. • Better than crossbar switch, but still big. • Another drawback: delay (due to the number of stages).
NUMA machine • To reduce delay, use NUMA (NonUniform Memory Access): • Some memory is associate with each CPU. Each CPU can access its own local memory quickly, but accessing anybody else’s memory is slower.
NUMA machine • Advantage: have better average access times than omega machines • Disadvantage: the placement of the programs and data becomes complicated because most access should go to the local memory.
Bus-Based Multicomputers Workstation Workstation Workstation Local memory Local memory Local memory CPU CPU CPU Network A multicomputer consisting of workstations on a LAN
Loosely-coupled software on loosely-coupled hardware • Network Operating Systems: A network of workstations connected by a LAN.
Tightly-coupled software on loosely-coupled hardware • True distributed systems: It is tightly-coupled software on the same loosely-coupled (multicomputer) hardware.
Characteristics of a distributed systems • There must be a single, global interprocess communication mechanism so that any process can talk to any other process. • Process management must also be the same everywhere. • The file system must look the same everywhere, too. • It is normal that identical kernels run on all the CPUs in the system. • Each kernel can have considerable control over its own local resources.
Tightly-coupled software on tightly-coupled hardware • Multiprocessor Timesharing Systems • The key characteristic of this class of system is the existence of a single run queue: a list of all the processes in the system that are logically unblocked and ready to run.
Design Issue - Flexibility • one school maintains that each machine should run a traditional kernel that provides most services itself. • The other maintains that the kernel should provide as little as possible, with the bulk of the operating system services available from user-level servers.
Design Issue - Reliability • High availability • Security • Fault tolerance
Design Issue - performance • Fine-grained parallelism: jobs that involve a large number of small computations, especially ones that interact highly with one another, may cause trouble on a distributed system with relatively slow computation. • Coarse-grained parallelism: jobs that involve large computations, low interaction rates, and little data.
Design Issue - Scalability • Potential bottlenecks that designers should try to avoid in very large distributed systems.
