27 Clock Synchronization

27 Clock Synchronization - Computer Science Clock...

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1 © Copyright Azer Bestavros / Al rights reserved. Computer Science CS-350 Clock Synchronization Azer Bestavros Computer Science Department Boston University Computer Science Clock Synchronization: Example Clock Synchronization: Example Impossible to guarantee that crystals in different computers all run at exactly the same frequency Î clock skew Î clock drifts Î problems! How should “make” interpret above timestamps? Same holds when using NFS mount Computer Science Physical Clocks Physical Clocks How do we measure time? Astronomers’ Answer: Look at the skies! ± 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 down! Also, there are short-term variations caused by turbulence deep in the earth’s core. ± Just use the mean over a long period… Computer Science Physical Clocks Physical Clocks How do we measure time? Physicists’ Answer: Look at an atom! ± Count the transitions of cesium 133 atom. ± 9,192,631,770 cesium transitions == 1 solar second. ± The Bureau Internationale de l’Heure (BIH) averages 50 reported clock ticks from various labs to produce the International Atomic Time (TAI). ± The TAI is the mean number of ticks of cesium 133 clocks since midnight on January 1, 1958 divided by 9,192,631,770 . ± 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, Colorado over shortwave radio station WWV. WWV broadcasts a short pulse at the start of each UTC second. [accuracy 10 msec.] ± GEOS (Geostationary Environment Operational Satellite) also offer UTC service. [accuracy 0.5 msec.] Computer Science Clock Synchronization Clock Synchronization Computer timers go off H times/sec, and increment the count of ticks (interrupts) since an agreed upon time in the past. This clock value is C. Using UTC time, the value of clock on machine p is C p (t). For perfect time, C p (t) = t & dC/dt = 1. For an ideal timer, H =60, should generate 216,000 ticks per hour; but typical errors of 10 –5 will make ticks vary from 215,998 to 216,002. This is called “clock drift”. Computer Science
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27 Clock Synchronization - Computer Science Clock...

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