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Wk6a

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Transcript

  • 1. Distributed Systems - An introduction
    • Introduction,
    • Features of Distributed Systems,
    • Naming,
    • Operating Systems,
    • Distributed Shared Memory,
    • Other Issues.
  • 2. Introduction
    • Computer systems developed from:
      • Standalone machines,
      • to direct communications between two machines,
      • to networks: one machine can communicate with any other networked machine.
    • BUT:
      • In all cases the user is always aware of the connection between machines,
      • Must issue explicit commands for the movement of data.
  • 3. Distribution(1)
    • Distributed Systems (DS) build on the networking layer:
      • Groups of independent machines acting together as one
        • Cooperate on a task, not just data sharing,
        • Distribute computation among several physical machines.
    • Distribution should be transparent to user AND programs at system call interface:
      • User or programmer should not be able to tell that a remote machine is involved,
      • IDEALLY DS should look like a conventional system to users:
        • Appear to users as a single computer .
  • 4. Distribution (2)
    • Simplest possible distributed system architecture:
      • Some processes provide services to others, known as servers,
      • Processes which use those services are called clients.
    • This approach known as client/server system.
    • What the sever does and any data it returns to the client varies depending on system requirements,
    • ALL different models of distributed computing can be reduced to this simple model.
  • 5. Features of Distributed Systems(1)
    • DS have features not found in standalone systems:
      • Economy and Performance
        • Price/performance ratio favours multiple small machines:
          • Especially commodity components,
          • See Top500 list in later slide,
          • Cumulative performance of micro computers at fraction of the cost of a main frame.
        • Utilise spare CPU cycles by dynamically using idle workstations. How many PCs in the Universities Labs? In effect free processing power!!!
        • Adapt to increased load and not collapse,
        • BUT more processors = more communications?: ISSUES HERE?!
  • 6. Features of Distributed Systems (2)
      • Reliability:
        • High reliability and fault tolerance (through redundancy, user need never know a problem occurred!).
      • Resource Sharing:
        • Economic: Share an expensive device (i.e Radio Telescope)
        • Convenience: not convenient to share a company’s database via floppy disk: distribution and data update problems!!!
      • Incremental Growth:
        • Not necessary to buy all processing power, memory, storage all at one time,
        • System expands to keep pace with growing demand.
  • 7. Example: Top 500 List (1)
    • Example of price/performance mentioned earlier.
    • Clusters are a type of distributed system.
    • Top500 List: lists sites operating the 500 most powerful computer systems ( http://www.top500.org/ ):
      • Compiled twice yearly.
      • Entry to top 10 positions requires > 9.8 TFlop/s (trillions of calculations per second).
      • #1: BlueGene/L DD2 beta, IBM/DOE, USA, 70.72 Tflop/s:
        • 32,768 0.7GHz PowerPC 440 CPUs,
      • #2: SGI Altix, Voltaire Infiniband, NASA, USA, 51.87 Tflop/s:
        • 10,160 1.5 GHz SGI Altix CPUs,
      • #3: Earth Simulator, Japan (Climate Modelling): 35.86 TFlop/s
        • 5,120 500 MHz NEC Vector CPUs, (640 8 CPU nodes), 10 TB memory, 16GB/s inter node bandwith,
        • Was #1 from 6/2002 until 11/2004 when BlueGene/L DD2 took over.
  • 8. Example: Top 500 List (2)
    • Highest ranking Intel cluster (at the moment):
      • #10: NCSA, Urbana-Champaign, USA,
      • 9.81 Tflop/s,
      • PowerEdge 1750,
      • 2500 P4 Xeon 3.06 GHz,
      • Myrinet interconnect.
    • Commodity hardware:
      • The same that you might buy for your own machines (Intel Xeon),
      • Myrinet probably not sitting on the shelf at your local PC World!
    • Economies of scale (COTs production):
      • Price/performance gains.
    • Operating System issues: How to coordinate various distributed components into a coherent system?!
  • 9. A Top 500 contender?
  • 10. Naming
    • Need to uniquely identify all resources
      • Individual machines, processes, files, printers, etc,
      • At system level identified by binary numbers,
      • Client needs to know identity of server machine AND identifier of process providing a service:
        • If server crashes and restarts: process may have a different process ID. Client unable to reach it.
        • If server machine crashes, another machine may take over servicing client requests. BUT client continues sending requests to old server’s binary address…
  • 11. Name server
    • At human level provide meaningful resource names:
      • i.e. LaserPrinter1 instead of 192.168.1.7 or the binary network address representation.
    • At machine level still identified by binary numbers,
    • Binding: The link between name and number,
    • Name server process:
      • Maintains a database of bindings,
      • Translates names to binary (or IP) addresses for the client,
      • If binary identifiers change then name server only needs updating,
      • If a client can locate the name server, it can locate any other resources in the system.
    • A single name can be used to reference a number of servers: Server fail-over, load-balancing, etc…
  • 12. Operating Systems
    • Two different types of underlying operating system have developed for use in distributed environments:
      • Network Operating System (NOS),
      • Distributed Operating System.
  • 13. Network Operating System
    • Attach file systems from a remote server onto a local machine:
      • Remote file system appears part of the local directory structure,
      • User sees no difference between local and remote files.
    • NOS only transfers portions (blocks) of a file that are actually in use,
    • If a file is modified: changes are written back to the server,
    • Similarity with paging,
    • NOS allows other resources like printers to appear local to the client.
  • 14. Distributed Operating System
    • True distributed OS must (at very least) begin to blur the boundaries between machines,
    • Still responsible for managing local resources:
      • CPU, network interface, etc.
    • But also responsible for:
      • Advertising resources to clients,
      • Export/import/schedule processes to/from other machines.
    • Should do all this TRANSPARENTLY!
    • Remote Procedure Calls (RPC) provide a way to transmit data between processes on different machines transparently:
      • Hides underlying socket communications.
  • 15. Hybrid Systems
    • Fully distributed OS not yet in wide spread use.
    • Usually single machine OS specially adapted:
      • BUT not fully transparent.
    • A number of approaches proposed as basis for future distributed operating systems
      • I.e. CORBA, Distributed Computing Environment
      • But what about other middleware: Jini, Jxta, Web Services?
  • 16. Distributed Shared Memory
    • Allow memory to be shared by processes on different machines.
    • Allows a shared memory programming model to be used by cooperating processes in distributed systems:
      • Using this model a standalone system could be distributed with minimum effort.
    • Transparent to the programmer:
      • Location of processes not relevant to programmer,
      • Can be on local or remote machines.
  • 17. Some Issues (1)…
    • Deadlock, and mutual exclusion, etc become more complicated in a distributed environment!
    • Distributed File systems:
      • Issues: data availability, transparency, caching, replication, consistency, ..
    • Failure recovery:
      • Underlying network, servers, processes may fail when least expected: redundancy, replication
      • How to keep the system operating to maximum effect in the face of adverse conditions?
      • Check pointing of computations?
      • Transactions must always be in a known state. How to ensure this?
    • Scalability
      • Does performance continue to grow when more machines are added or is there a saturation point? How do we overcome this?
  • 18. Summary
    • A very brief look at some high-level issues associated with distributed (operating) systems.
    • Main concepts:
      • Price, performance,availability, redundancy, reliability, sharing, transparency, scalability.