[Python-Dev] PEP 418 is too divisive and confusing and should be postponed

Steven D'Aprano steve at pearwood.info
Sat Apr 7 12:40:46 CEST 2012


Victor Stinner wrote:
> 2012/4/7 Janzert <janzert at janzert.com>:
>> On 4/5/2012 6:32 AM, Victor Stinner wrote:
>>> I prefer to use CLOCK_MONOTONIC, not because it is also available for
>>> older Linux kernels, but because it is more reliable. Even if the
>>> underlying clock source is unstable (unstable frequency), a delta of
>>> two reads of the CLOCK_MONOTONIC clock is a result in *seconds*,
>>> whereas CLOCK_MONOTONIC_RAW may use an unit a little bit bigger or
>>> smaller than a second. time.monotonic() unit is the second, as written
>>> in its documentation.
>> I believe the above is only true for sufficiently large time deltas. One of
>> the major purposes of NTP slewing is to give up some short term accuracy in
>> order to achieve long term accuracy (e.g. whenever the clock is found to be
>> ahead of real time it is purposefully ticked slower than real time).
> 
> I don't think that NTP works like that. NTP only uses very smooth adjustements:
> 
> ""slewing": change the clock frequency to be slightly faster or slower
> (which is done with adjtime()). Since the slew rate is limited to 0.5
> ms/s, each second of adjustment requires an amortization interval of
> 2000 s. Thus, an adjustment of many seconds can take hours or days to
> amortize."
> http://www.python.org/dev/peps/pep-0418/#ntp-adjustment


That is incorrect. NTP by default will only slew the clock for small 
discrepancies. For large discrepancies, it will step the clock, causing the 
time to jump. By default, "large" here means more than 128 milliseconds.

Yes, milliseconds.

http://www.ntp.org/ntpfaq/NTP-s-config-tricks.htm#AEN4249


In any case, NTP is not the only thing that adjusts the clock, e.g. the 
operating system will adjust the time for daylight savings.


-- 
Steven



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