On 2011-04-18, Chris Albertson <[email protected]> wrote: > On Sun, Apr 17, 2011 at 11:33 PM, unruh <[email protected]> wrote: > >>> NTP works kind of like that. ?It uses a set of reference clocks and >>> watches the rate of your local clock relative to the reference >>> clock(s) and depending on details it make time some time >> >> Nice analogy. If only chrony did not demonstrate that using exactly the >> same time interchange with an accurate clock, the computer can achieve >> usec accuracy within less than an hour. Ie, your analogy simply does not >> hold as a general statement. > > The analogy is perfect and applies to all clocks, computer based and > otherwise. There is simply no way to match the rate of one clock to > another without waiting until you are able to measure divergence. The > length of time you have to wait depends on the accuracy which which > you can measure the divergence and your goal for precision. It > general you devide one into the other to get time. Now if you have > two programs and one is faster at setting the clocks rate, then you > have an implementation detail. What I said above defines the minimum > time required. Of course one would work slower. The OP's question > was why in theory does it take a long time, and gave no details of his > setup.
Since you can measure the time to usec, in 1 sec you can measure rate offsets of 1PPM and offsets of 1usec. Thus. with your arguments you should be able to get the clock to within 1PPM and 1usec within 1 sec., and .1PPM within 10 sec. This is nowhere near as fast as ntpd works. And the OP did give details of his setup-- he was refering to experiments done with ntpd which showed that it took 10 hr to recover from a 40PPM rate change in the system clock (those experiments used a GPS clock as the time source, with a interrupt service routing with directly timestamped the interrupt immediately as it came in, so noise in the external clock was not a problem). _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
