LM.Lecture7 - Distributed Operating Systems CS551 Colorado...

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Distributed Operating Systems CS551 Colorado State University at Lockheed-Martin Lecture 7 -- Spring 2001
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28 March 2001 CS-551, Lecture 7 2 CS551: Lecture 7 Topics Distributed Synchronization (Chapter 10) Global Time Physical Clocks Network Time Protocol (NTP) Logical Clocks
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28 March 2001 CS-551, Lecture 7 3 Global Clocks Distributed systems have no global clock Each processor in the system is autonomous Each processor has its own clock Impossible to have the processes across the system synchronized exactly Cannot know the true time order of any two events
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28 March 2001 CS-551, Lecture 7 4 Ordering of Events It is impossible to know which of two events happens first This has an impact on scheduling This makes the distributed system harder to debug
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28 March 2001 CS-551, Lecture 7 5 Global Time via Shared Memory? Distributed systems have no shared memory Thus it is hard (impossible) to get an up-to- date state of the entire system A global state would give us A view of all local states The contents of all messages currently in transit
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28 March 2001 CS-551, Lecture 7 6 Definitions Drifting : “the gradual misalignment of once synchronized clocks caused by the slight inaccuracies of the time-keeping mechanisms” (Galli) Clock Skew : “the difference in time between two clocks due to drifting” (Galli)
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28 March 2001 CS-551, Lecture 7 7 Global Time via Physical Clocks? Universal Time Coordinator (UTC) A consensus Accurate to +/- 1 second per 20,000,000 years about 1 part in 10^12 Sources: Geostationary Operational Environmental Satellites (GEOS) GPS devices WWV: a Fort Collins radio station
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28 March 2001 CS-551, Lecture 7 8 Global Time via WWV A Fort Collins shortwave radio station Transmits UTC signal Low-frequency => less atmospheric disturbance 2000 mile radius Sends signals once a day to clocks/watches Transmission delay is 24000 microseconds at the extreme range Less than 0.1 second Can be corrected for
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28 March 2001 CS-551, Lecture 7 9 UTC Time Providers Time Provider : “a commercial device that is capable of directly receiving information from a UTC server and making appropriate adjustments due to communication delays” (Galli) Such devices are currently installed in watches, clocks, and computers
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28 March 2001 CS-551, Lecture 7 10 Figure 10.1 Network Delays when Communicating Time. (Galli,p.239)
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28 March 2001 CS-551, Lecture 7 11 Correcting for Transmitted Time A UTC signal is sent out Transmit time varies depending on Atmospheric conditions Humidity Receiving clock must make compensation for transmit time However, once reset, clock will start drifting again
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28 March 2001 CS-551, Lecture 7 12 Figure 10.2 Forward Adjustment of a Clock. (Galli,p.240)
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LM.Lecture7 - Distributed Operating Systems CS551 Colorado...

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