Clocks - Today Time Synchronization CSCI 5105 Foundations of Modern Operating Systems Event Ordering Physical Clocks Logical Clocks Instructor

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1 CSCI 5105 Foundations of Modern Operating Systems Instructor: Abhishek Chandra 2 Today Time Synchronization Physical Clocks Event Ordering Logical Clocks 3 Synchronization Agreement over global state among distributed servers/processes Communication: Different processes must see messages and events in a consistent order Sharing: Shared resources/data should be consistent Master/slave relation: Many distributed algorithms require a master server 4 Time Synchronization Uniprocessors Single clock All processes see the same time Distributed Systems Different clocks Each machine sees different times
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2 5 Clocks and Clock Drifts Clocks are oscillators Drift caused by differences in oscillator frequencies Coordinated universal time (UTC) International standard based on atomic time Broadcast via radio, satellites 6 Clock Synchronization Each clock has a maximum drift rate r 1- r <= dC/dt <= 1+ r Two clocks may drift by 2 r D t in time D t To limit drift to d => resynchronize every d/ 2 r seconds 7 Cristian’s Algorithm Time server: coordinated with the UTC Each machine asks for current time periodically Time server returns its current time
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This note was uploaded on 10/21/2011 for the course CSCI 5105 taught by Professor Staff during the Spring '08 term at Minnesota.

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Clocks - Today Time Synchronization CSCI 5105 Foundations of Modern Operating Systems Event Ordering Physical Clocks Logical Clocks Instructor

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