[ntp:questions] ntp discipline of local time?
unruh-spam at physics.ubc.ca
Tue Mar 25 20:27:41 UTC 2008
"David L. Mills" <mills at udel.edu> writes:
>The kernel discipline is almost identical to the daemon discipline with
>the exception that the fancy code to combine the PLL and FLL near the
>Allan intercept is absent. Without the PPS signal, the discipline
>behaves as a second-order loop; with the PPS it behaves as two separate
>first-order loops, one for phase, the other for frequency. When
>necessary to set the freque directly, ther kernel is use for that. Note
>that the peripheral functions, like the clock state machine and
>poll-adjust algorithm continue in the daemon.
Let me concentrate on the situation without the PPS signal.
Is the code in ntp_loopfilter.c the code which disciplines the clock?
If so, then what it reads like to me is that ntp_adjtime (adjtimex in
Linux) is used to alter the clock frequency each time a valid offset is
delivered from the clock filter/peer finder routines. That frequency
adjustment is a multiple of that offset (that multiple depending on the
poll interval, etc) using the ntp_adjtime routine. Then that offset itself
is driven to zero by using adjtime once per second and adjusting the clock
by a small multiple of that offset. (something like 1/(16 times the poll
interval) ) (let me not worry about the allan intercept stuff for now).
Is my reading of the code correct? What I describe would, in the long term
be a simple second order loop-- its interaction with the "clock-filter"
algorithm is rather more complex than that, but lets leave that aside for
now. Also the fact that the adjtime part of the routine simply throws away
the part of the previous adjustment that was not completed-- I assume this
is done because it almost never actually has any uncompleted adjustment.
Is there anything else that is going on? Is "kernel discipline" anything
more than the ability to change the frequency of the clock ( or rather
change the factor that translates the CPU cycle counter to time)?
Sorry to ask such detailed questions, but I am having a hard time
disentangling the code to see what is really happening.
>> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>>>>How does ntp actually discipline the local clock? I have a gps received
>>>If you are using the kernel time discipline, which you should be using
>>>for high accuracy, nptd doesn't discipline the clock; it is the kernel
>>>code that does that, based on measurements provided by ntpd.
>> I do not think that this is right, unless you are referring to a PPS
>> sounce. ntp sets the frequency of the kerhel clock (Is that change in
>> frequency what you mean by kernel time discipline) by a very simple second
>> order PDE feedback, and the offset by and exponential first order feedback
>> scheme. At least that is what it looks like to me trying to read
>>>>attached to a computer which is disciplined by a remote clock over an ADSL
>>>>line. (Ie, the gps does not act as a refclock -- it is purely to measure
>>>>the actual offset of the system. It is only the remote server that actaully
>>>>acts the ntp reference source.)
>>>>I can watch how ntp alters the local clock in response to remote
>>>>offsets. The response is not linear. rather it is "curved" as though the
>>>>rate of the local clock were exponentially eliminating the offset. But this
>>>That sounds very plausible. The clock discipline code solves for both
>>>frequency and phase errors. The phase error is probably being filtered
>>>using an IIR filter, and that is what you are seeing, and also the
>>>mechanism ntpd uses to stop wandering off if it stops receiving updates
>>>(the frequency measurement error can produce unbounded phase errors, but
>>> the phase error correction is bounded).
>>>>is between two succesive runnings of the loopstats. Where is this behaviour
>>>>determined? -- ie which routines determines the response of the system
>>>>between to successive measurements of the offset?
>>>If you don't use the kernel discipline, on Unix-like systems, it will
>>>implement the same filters in user space and apply phase adjustments at
>>>each kernel update. For ntpv3, those updates were every 4 seconds; for
>>>ntpv4, I believe it does them every second. A normal Unix-like system
>>>will implement the phase change by increasing or decreasing the amount
>>>by which the software clock is updated for every tick by +/- 500ppm,
>>>until the adjustment is complete.
>> It is the linux system I am interested in. It looks to me like it adjusts
>> the frequency with a simply second order feedback loop using the
>> ntp_adjtime system call, and then drives the
>> offset to zero with an exponential run once a second (?? I cannot
>> disentangle the code to really be sure of this) using the adjtime system
>> call. That exponential has a huge time constant-- something like 16 times
>> the poll interval.
>>>Windows has a different kernel interface, and I believe that ntpd
>>>modulates the effective length of a tick.
>>>Note, in spite of what other replies may imply, the physical clock
>>>frequency is never actually changed; what is actually changed is the
>>>amount by which the software clock is incremented for ever n-cycles of
>>>whatever is used for the reference frequency.
>> Of course. There is no way that the physical clock can be influenced by
>> software. The system simply changes the relation between harware cpu cycle
>> counts and time.
>>>If you want the actual code and fine details, you will be able to find
>>>them as easily as I will, so I'll leave that as an exercise for the reader.
>> I guess I was hoping that perhaps the person/people who actually wrote the
>> code could tell me what was going on in the code. While the code is
>> reasonably annotated, those annotations do not give me at least a good
>> sense of the overall picture.
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