lect-08 - Computer Science 425 Distributed Systems Lecture...

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Lecture 8- 1 2006, ,2007, M. T. Harandi Computer Science 425 Distributed Systems Lecture 8 Mutual Exclusion: Coordination
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Lecture 8- 2 2006, ,2007, M. T. Harandi Critical section problem: Mutual exclusion is required to prevent interference and ensure consistency when accessing the resources. Solutions: Semaphores, mutexes, etc. in local operating systems Message-passing-based protocols in distributed systems: enter() the critical section AccessResource() in the critical section exit() the critical section Distributed mutual exclusion requirements: Safety - At most one process may execute in CS at any time Liveness – Every request for a CS is eventually granted Ordering (desirable) – Requests are granted in FIFO order Mutual Exclusion
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Lecture 8- 3 2006, ,2007, M. T. Harandi Performance Evaluation Criteria Bandwidth : the number of messages sent in each entry and exit operation. Client delay : the delay incurred by a process at each entry and exit operation. Synchronization delay : the time interval between one process exiting the critical section and the next process entering it. This is also termed as throughput -- the rate at which the processes can access the critical section. For all the algorithms studied, we make the following assumptions: Each pair of processes is connected by reliable channels (such as TCP). Messages are eventually delivered to recipients’ input buffer. Processes will not fail.
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Lecture 8- 4 2006, ,2007, M. T. Harandi A central coordinator Is appointed or elected Ensures only one thread at a time can access the CS Operations: On exiting the CS, send a message to the server to release the token. Upon receipt of a request, if no other process has the token, the server replies with the token; otherwise, the server queues the request. Upon receipt of a release message, the server removes the oldest entry in the
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lect-08 - Computer Science 425 Distributed Systems Lecture...

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