Communications of the ACM
Time Is an Illusion Lunchtime Doubly So
Credit: Stokkete
One of the more surprising things about digital systems—and, in particular, modern computers—is how poorly they keep time. When most programs ran on a single system this was not a significant issue for the majority of software developers, but once software moved into the distributed-systems realm this inaccuracy became a significant challenge. Few programmers have read the most important article in this area, Leslie Lamport's 1978 Communications article "Time, Clocks, and the Ordering of Events in a Distributed System,"2 and only a few more have come to appreciate the problems they face once they move into the world of distributed systems.
Any discussion of time should center around two different measurements: synchronization and syntonization. Synchronization, loosely defined, is how close two different clocks are to each other at any particular instant. If two clocks, or computers, claim it is 15:30 at exactly the same moment, then they are considered to be in sync. The definition of "exactly the same moment" is where the first difficulty arises. Do you care about the same minute (15:30), the same second (15:30:00), millisecond, microsecond, nanosecond? The level of accuracy you wish to achieve defines, approximately, the level of difficulty in attaining it.
Comments
CACM Administrator
The following letter was published in the Letters to the Editor of the April 2016 CACM (http://cacm.acm.org/magazines/2016/4/200162).
--CACM Administrator
I really enjoyed George V. Neville-Neil's article "Time Is an Illusion Lunchtime Doubly So." (Jan. 2016) and am a big fan of his Kode Vicious columns as well. However, I could not fathom the intriguing figure (see it here) in the article, labeled simply "PTP log graph," nor could I find any reference to it in the text that might shed light on what property of the Precision Time Protocol it is supposed to illustrate. Not being an electrical engineer myself, I thought it might be something obvious to someone in the field, yet a friend with a Ph.D. in electrical engineering was equally flummoxed. It was such an interesting chart I am keen to understand what it means if I can. Prof. Neville-Neil, can you enlighten me?
John Beresniewicz
Half Moon Bay, CA
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AUTHOR'S RESPONSE
The PTP Log Graph in the figure showed the offset of a system clock that is not regulated by an outside time source (such as NTP and PTP). Without an outside time source, the clock wanders away from where we would expect it to be if the system's crystal oscillator was more stable, which it is not.
George V. Neville-Neil
Brooklyn, NY
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