Distributed Operating Systems Andy Wang COP 5911 Advanced Operating Systems Outline Introductory material Distributed IPC Distributed file systems Security for distributed systems Outline of Introductory Materials Why distributed operating systems Important issues in distributed OSes Important distributed OS tools and mechanisms Why Bother Economics of hardware Local autonomy Resource sharing Effective use of networks Reliability Economics of Hardware Cheaper to build many small machines than one large one Due to Economics of scale Chip design and fabrication issues Gives purchasers easy options to increase computer power Local Autonomy Single user machines better suited for most computer tasks Allow dedication of resources to a user s task E g easier to guarantee response time Owning user can control his computer power Resource Sharing But users need to share resources Hardware resources Printers and tape drives Software resources Data Access to software services Network Usage Users often want to communicate With other local users And to make data available to world System needs to support user interactions Generally demands cooperation among multiple machines Reliability Failure of a single machine no longer halts everyone Generally graceful degradation of the overall system s resources Ability to apply fault tolerance for important tasks at a high architectural level Problems with Distributed Systems More complex model of the system Harder to provide correct operation Harder to allocate resources properly Security Dealing with partial failures Scaling issues Heterogeneity Complexity of the Model Problem for Designers Users System software Harder to understand what will happen at any given case Harder to design software to handle even understood complexities Difficulties with Correct Operation Distribution requires more complex synchronization Differences between similar operations with remote and local New sources of nonuniform timings Difficulties of Allocating Resources Local machine may have inadequate resources for a task While a remote machine lies idle Infeasible to control resources centrally Do I need to go remote to satisfy malloc Using remote resources conflicts with local autonomy Security Security problems much trickier when no centralized control Data communications more subject to eavedropping Physical security measures typically infeasible for many problems In very wide distributed systems very tricky problems Dealing with Partial Failures Single machines usually have easy failure modes Distributed systems face complications Even detecting failure of a remote machine is nontrivial E g what s the difference between a slow network a failed network and a crashed machine Scaling Issues Distributed systems control much larger pools of resources So algorithms that scale well become much more important Scaling puts severe limits on close cooperation Heterogeneity Problems Most distributed systems must address problems of differing hardware and software Problems with data formats executable formats Problems with software versioning Problems with different OSes Resource Sharing Resource sharing helps with some of the problems Motivations for resource sharing Information exchange Load distribution Computational parallelism The fundamental distributed system problem Distribution Complicates Everything Process control and synchronization Interprocess communications File systems Security Device management Important Research Areas in Distributed Operating Systems In the area of processes Remote interprocess communications Synchronization Naming Distributed process management More Research Areas In the area of resource management Resource allocation Distributed deadlock mechanisms Protection and security Managing communication resources Taxonomy of Distributed Systems Data Stream Single Multiple Single SISD SIMD Multiple MISD MIMD Instruction Stream Network OSes vs Distributed OSes Network Oses control a single machine plus some remote access facilities Distributed OSes control a collection of machines Not a hard and fast distinction Network OS Diagram Network OS Network OS Network OS Network OS Network OS Distributed OS Diagram Network OS Network OS Distributed Operating system Network OS Network OS Network OS Characteristics of Network OSes Private per machine OS Normal operations only on local machine Machine boundaries are explicit Little per user fault tolerance Characteristics of Distributed OSes Single system controls multiple machines Use of remote machines invisible Users treat system as virtual uniprocessor Strong fault tolerance Reality is Somewhere in Between Relatively few true distributed OSes Network OS model But many modern systems have distributed OS like capabilities And they also support network OS operations Like remote file access Like rlogin and remote shell WWW access is in between The Role of the Network Distributed OSes made possible by network Two fundamental types Local area networks Long haul networks With very different characteristics Local Area Networks High bandwidth Low delay Shared by modest number of machines Covers modest geographical area Dedicated to small group of users Can be regarded as extension to computer s backplane Long Haul Networks Lower bandwidth Longer delays Shared by large numbers of machines Covers very wide area Typically shared by many independent groups Communication Protocols Well defined methods of intermachine data exchange To automatically handle problems of connecting network Many different types required available Using Protocols in Distributed Operating Systems Any intermachine operation requires a protocol to control it So all machines involved can understand data exchange Fundamental choice General vs special purpose protocols General vs Special Purpose Protocols General protocols try to handle any kind of traffic Special purpose protocols are customized for one situation General protocols simplify everything Special purpose protocols may perform better Important Issues in Distributed Operating Systems Communication model Process interaction Transparency Heterogeneity Autonomy Consistency and transactions Communication Models for Distributed Operating Systems How do machines communicate Generally message based at some level ISO model adds too much overhead So special purpose protocols or simplified protocol stacking model is typically used Process Interaction in Distributed Operating Systems How do processes
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