27 Clock Synchronization

27 Clock Synchronization - Computer Science Clock...

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1 © Copyright Azer Bestavros / All 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 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 Clock Synchronization Clock Synchronization Manufacturer specs give the maximum drift rate ( ρ ).
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