[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
David L. Mills
mills at udel.edu
Fri Jan 25 01:02:00 UTC 2008
David,
1. I have explained in very gory detail in many places how the time
constant is chosen for the best accuracy using typical computer
oscillators and network paths. See the briefings on the NTP project page
and especially the discussion about the Allan intercept. If you want the
best accuracy over the long term, you had better respect that. Proof
positive is in my 1995 SIGCOMM paper, later IEEE Transactions on
Networking paper and das Buch. I abvsolutely relish scientific critique,
but see the briefings and read the papers first.
2. To reduce the convergence time, reduce the time constant, but only at
the expense of long term accuracy. An extended treatise on that is in
das Buch, especially Chaptera 4, 6 and 12. I would be delighted to hear
critique of the material, but read the chapters first.
3. The reference implementation has a lower poll interval limit of 16 s
due to interactions with the 1-s clock amortization interval. In
principle, the time constant could be reduced to anything you want using
the kernel support.
4. At the lower poll interval/time constant the NTP loop degenerates to
a type-II linear phase-lock loop, the characteristics of which are very
well known to communications engineers. The PLL that drives your
computer processor clock is in fact a type-III loop, although it might
not be strictly linear. I like linear loops; I can describe them
mathematically, predict precisely their behavior and how to manage the
risetime and overshoot.
5. This flap about the speed of convergence has become silly. Most of us
are less concerned about squeezing to the low microseconds in four
minutes than long term accuracy over sometimes raunchy communication
paths. If this is not the case, then for heaven's sake use crony and get
off the bus. I see my stop coming up...
Dave
David Woolley wrote:
> David L. Mills wrote:
>
>>
>> The NTP discipline is basically a type-II feedback control system.
>> Your training should recall exactly how such a loop works and how it
>> responds to a 50-ms step. Eleven seconds after NTP comes up the
>> mitigation
>
>
> You both have problems here.
>
> Dave Mills: your problem is that you haven't explained why one should
> continue to use a long time constant linear feedback system when a human
> observer can easily tell you how to get within 10 microseconds of the
> correct time after no more than about 3 samples.
>
> Bill Unruh: you haven't explained what real world situation this test
> is simulating; it is a standard doctrine that ntpd is not a substitute
> for good hardware and system software (e.g. you shouldn't use ntpd to
> get round lost clock interrupts).
>
>> algorithms present that transient to the loop and what happens
>> afterwards conforms to the equations of control theory. Discussion
>> about what happens at any time after that is a matter of mathematics
>> and ntpd does conform to the mathematics as confirmed by observation
>> and simulation.
>
>
> That's an indication that the equations are inappropriate in that context.
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