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23-Distributed_Systems_I - CSE 421/521 Operating Systems...

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1 CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ş ar University at Buffalo November 22 nd , 2011 Lecture - XXIII Distributed Systems - I Motivation Distributed system is collection of loosely coupled processors that do not share memory interconnected by a communications network Reasons for distributed systems Resource sharing • sharing and printing files at remote sites • processing information in a distributed database • using remote specialized hardware devices Computation speedup – load sharing Reliability – detect and recover from site failure, function transfer, reintegrate failed site Communication – message passing Distributed-Operating Systems • Users not aware of multiplicity of machines Access to remote resources similar to access to local resources • Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task • Computation Migration – transfer the computation, rather than the data, across the system Distributed-Operating Systems (Cont.) • Process Migration – execute an entire process, or parts of it, at different sites Load balancing – distribute processes across network to even the workload Computation speedup – subprocesses can run concurrently on different sites Hardware preference – process execution may require specialized processor Software preference – required software may be available at only a particular site Data access – run process remotely, rather than transfer all data locally Network Topology Robustness in Distributed Systems • Failure detection • Reconfiguration
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Failure Detection Detecting hardware failure is difficult To detect a link failure, a handshaking protocol can be used Assume Site A and Site B have established a link At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running
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23-Distributed_Systems_I - CSE 421/521 Operating Systems...

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