Distributed SystemsIntroduction to Distributed SystemsAdvantages of Distributed Systems over Centralized SystemsAdvantages of Distributed Systems over Independent PCsDisadvantages of Distributed SystemsHardware ConceptsBus versus Switched MIMDSwitched MultiprocessorsMulticomputersSoftware ConceptsNetwork Operating SystemsNFS(True) Distributed SystemsMultiprocessor Operating SystemsDesign Issues of Distributed Systems1. TransparencyTypes of transparency2. Flexibility3. Reliability4. Performance5. Scalability11/14/00 CSE 380 1University of PennsylvaniaDistributed SystemsCSE 380Lecture Note 13Insup Lee11/14/00 CSE 380 2University of PennsylvaniaIntroduction to Distributed Systems Why do we develop distributed systems?•availability of powerful yet cheap microprocessors (PCs, workstations), continuing advances in communication technology, 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 system. Examples:•Network of workstations•Distributed manufacturing system (e.g., automated assembly line)•Network of branch office computers11/14/00 CSE 380 3University of PennsylvaniaAdvantages of Distributed Systems over Centralized Systems•Economics: a collection of microprocessors offer a better price/performance than mainframes. Low price/performance ratio: cost effective way to increase computing power.•Speed: a distributed system may have more total computing power than a mainframe. Ex. 10,000 CPU chips, each running at 50 MIPS. Not possible to build 500,000 MIPS single processor since it would require 0.002 nsec instruction cycle. Enhanced performance through load distributing.•Inherent distribution: Some applications are inherently distributed. Ex. a supermarket chain.•Reliability: If one machine crashes, the system as a whole can still survive. Higher availability and improved reliability.•Incremental growth: Computing power can be added in small increments. Modular expandability•Another deriving force: the existence of large number of personal computers, the need for people to collaborate and share information.11/14/00 CSE 380 4University of PennsylvaniaAdvantages of Distributed Systems over Independent PCs•Data sharing: allow many users to access to a common data base•Resource Sharing: expensive peripherals like color printers•Communication: enhance human-to-human communication, e.g., email, chat•Flexibility: spread the workload over the available machines11/14/00 CSE 380 5University of PennsylvaniaDisadvantages of Distributed Systems•Software: difficult to develop software for distributed systems•Network: saturation, lossy transmissions•Security: easy access also applies to secrete data11/14/00 CSE 380 6University of PennsylvaniaHardware Concepts Taxonomy (Fig. 9-4) MIMD (Multiple-Instruction Multiple-Data) Tightly Coupled versus Loosely Coupled-Tightly coupled systems (multiprocessors)oshared memoryointermachine delay short, data rate high-Loosely coupled systems (multicomputers)oprivate memoryointermachine delay long, data rate low11/14/00 CSE 380 7University of PennsylvaniaBus versus Switched MIMD•Bus: a single network, backplane, bus, cable or other medium that connects all machines. E.g., cable TV•Switched: individual wires from machine to machine, with many different wiring patterns in use. Multiprocessors (shared memory)–Bus–Switched Multicomputers (private memory)–Bus–Switched11/14/00 CSE 380 9University of PennsylvaniaSwitched Multiprocessors Switched Multiprocessors (Fig. 9-6)-for connecting large number (say over 64) of processors-crossbar switch: n**2 switch points-omega network: 2x2 switches for n CPUs and n memories, log n switching stages, each with n/2 switches,-total (n log n)/2 switches -delay problem: E.g., n=1024, 10 switching stages from CPU to memory. a total of 20 switching stages. 100 MIPS 10 nsec instruction execution time need 0.5 nsec switching time-NUMA (Non-Uniform Memory Access): placement of program and data-building a large, tightly-coupled, shared memory multiprocessor is possible, but is difficult and expensive11/14/00 CSE 380 10University of PennsylvaniaMulticomputers Bus-Based Multicomputers (Fig. 9-7)-easy to build-communication volume much smaller-relatively slow speed LAN (10-100 MIPS, compared to 300 MIPS and up for a backplane bus) Switched Multicomputers (Fig. 9-8)-interconnection networks: E.g., grid, hypercube-hypercube: n-dimensional cube11/14/00 CSE 380 11University of PennsylvaniaSoftware Concepts•Software more important for users•Three types:1. Network Operating Systems2. (True) Distributed Systems3. Multiprocessor Time Sharing11/14/00 CSE 380 12University of PennsylvaniaNetwork Operating Systems-loosely-coupled software on loosely-coupled hardware-A network of workstations connected by LAN-each machine has a high degree of autonomyorlogin machineorcp machine1:file1 machine2:file2-Files servers: client and server model-Clients mount directories on file servers-Best known network OS:oSun’s NFS (network file servers) for shared file systems (Fig. 9-11)-a few system-wide requirements: format and meaning of all the messages exchanged11/14/00 CSE 380 13University of PennsylvaniaNFS NFS Architecture•Server exports directories•Clients mount exported directories NSF Protocols•For handling mounting•For read/write: no open/close, stateless NSF Implementation11/14/00 CSE 380 14University of Pennsylvania(True) Distributed Systemstightly-coupled software on loosely-coupled hardwareprovide a single-system image or a virtual uniprocessora single, global interprocess communication mechanism, process management, file system; the same system call interface everywhereIdeal definition: “ A distributed system runs on a collection of computers that do not have shared memory, yet looks like a single computer to its users.”11/14/00 CSE 380 15University of PennsylvaniaMultiprocessor Operating Systems (Fig. 9-12)-Tightly-coupled software on tightly-coupled hardware-Examples: high-performance servers-shared memory-single run queue-traditional file system as on a single-processor system: central block cache Fig. 9-13 for comparisons11/14/00 CSE 380 16University of PennsylvaniaDesign Issues of Distributed Systems•Transparency•Flexibility•Reliability•Performance•Scalability11/14/00 CSE 380 17University of Pennsylvania1. Transparency•How to achieve the single-system