Synch - Synchronization Part1 of Tanenbaums...

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  1 Synchronization Part 1 REK’s adaptation of  Claypool’s adaptation  of Tanenbaum’s Distributed Systems Chapter 5
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Distributed Computing Systems 2 Outline  Clock Synchronization Clock Synchronization Algorithms Logical Clocks Election Algorithms Mutual Exclusion Distributed Transactions Concurrency Control
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Distributed Computing Systems 3 Clock Synchronization make  example When each machine has its own clock, an event  that occurred after another event may nevertheless  be assigned an earlier time. Same holds when using NFS mount Can all clocks in a distributed system be synchronized?
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Distributed Computing Systems 4 Physical Clocks It is impossible to guarantee that crystals in different computers  all run at exactly the same frequency. This difference in time  values is  clock skew . “Exact” time was computed by astronomers The difference between two transits of the sun is termed a  solar  day.  Divide a solar day by 24*60*60 yields a  solar second. However, the earth is slowing! (35 days less in a year over  300 million years) There are also short-term variations caused by turbulence  deep in the earth’s core. A large number of days  (n)  were used used to the average day  length, then dividing by 86,400 to determine the  mean solar  second .
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Distributed Computing Systems 5 Physical Clocks  Computation of the mean solar day.
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Distributed Computing Systems 6 Physical Clocks Physicists take over from astronomers and count the  transitions of cesium 133 atom 9,192,631,770 cesium transitions == 1 solar second 50 International labs have cesium 133 clocks. The  Bureau Internationale de l’Heure (BIH) averages  reported  clock ticks to produce the  International Atomic Time  ( TAI ) . The  TAI  is mean number of ticks of cesium 133 clocks  since midnight on January 1, 1958 divided by  9,192,631,770 .
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Distributed Computing Systems 7 Physical Clocks To adjust for lengthening of mean solar day,  leap  seconds  are used to translate TAI into  Universal Coordinated Time ( UTC ) . UTC is broadcast by NIST from Fort Collins, 
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This note was uploaded on 11/15/2011 for the course ECKOW 123 taught by Professor Riuyt during the Spring '11 term at University of Damascus.

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Synch - Synchronization Part1 of Tanenbaums...

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