[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Fri Jan 25 17:53:41 UTC 2008


"David L. Mills" <mills at udel.edu> writes:

>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 

The Allan intercept is predicated on a very specific model of the noise in
a clock ( as I recall basically random gaussian noise at high frequencies,
and 1/f noise at low). It is not at all clear that real computers comply
with that. 


>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.

While you may know what in the world Das Buch is (Hitlers Mein Kampf?) I do
not. Nor do I know where to get it. 



>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.

Of course they are. That is not the question. The question is is that the
best you can do?


>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...

Sure. To an extent this argument is "academic" in that most of us do not
need even msec accuracy. But "good enough" and "best" are not necessarily
synonymous.



>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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