NTP Polling Explanation - NTP

This is a discussion on NTP Polling Explanation - NTP ; I have System Administrators who insist upon maxpoll=4. First of all, thanks to those in this list who I have plagerized! Second, is the following explanation accurate and use appropriate terminology. Q: Do you recommend any tweaks to the NTP ...

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Thread: NTP Polling Explanation

  1. NTP Polling Explanation

    I have System Administrators who insist upon maxpoll=4. First of all,
    thanks to those in this list who I have plagerized! Second, is the
    following explanation accurate and use appropriate terminology.

    Q: Do you recommend any tweaks to the NTP configuration beyond the listing
    of the servers, such as maxpoll=4?

    A: Tweaks are not recommended and are discouraged. Forcing a poll
    interval that is more frequent than NTP would normally select on its own
    hurts accuracy and stability of time on the local system. NTP polling does
    not directly synchronize the local system clock to the server clock; rather,
    a complex algorithm calculates an adjustment value for each tick of the
    local system clock. Shorter polling intervals cause NTP to make large but
    less accurate calculations that never stabilize, causing the local system
    clock to wander. Longer polling intervals allow NTP to calculate smaller
    tick adjustments that stabilizes to a more accurate value, reducing wander
    in the local system clock.

    NTP dynamically selects the optimal poll interval between the values of
    minpoll and maxpoll, which default to 64 and 1024 seconds respectively and
    are correct for most environments. Shorter values are used to correct large
    errors while longer values are used to refine accuracy. NTP dynamically
    varies this value as conditions change and requires several days of
    continuous operation to converge on an optimal adjustment value.


    Thanks in advance ...
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  2. Re: NTP Polling Explanation

    In article <49390eba0608150658o16896df0ra6eb1f98812b1b42@mail. gmail.com>,
    whmyers@gmail.com (Bill Myers) wrote:

    > local system clock. Shorter polling intervals cause NTP to make large but
    > less accurate calculations that never stabilize, causing the local system
    > clock to wander. Longer polling intervals allow NTP to calculate smaller


    It would take some time to read the code and spec closely enough to be
    completely sure, but I believe that maxpoll doesn't actually cap the
    loop time constant, so the loop is still less responsive to jitter. However,
    by setting maxpoll you are wasting resources by oversampling at a rate that
    is simply filtered out by the low pass effect of the control loop. Also,
    ntpd chooses the sample from the last 8 that has the tightest error bounds,
    where the bounds increase with time as well as depending on things like
    round trip time. If there is an 8 minute period with extended round trip
    times because of an assymetric traffic load, maxpoll=4 will cause an
    ureliable sample to be used, but the default polling interval will reject
    that period.

    As I understand, if the poll rate is faster than required, the sensitivity
    will be reduced to compensate.

    I think the one advantage of limiting maxpoll is that the system will detect
    a step-out (>128ms) condition faster, but its better if the system doesn't
    get those in the first place.

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