[Python-Dev] PEP 418 is too divisive and confusing and should be postponed (original) (raw)

Victor Stinner victor.stinner at gmail.com
Sun Apr 8 00:24:16 CEST 2012

2012/4/7 Steven D'Aprano <steve at pearwood.info>:

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 CLOCKMONOTONIC, 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 CLOCKMONOTONIC clock is a result in seconds, whereas CLOCKMONOTONICRAW 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

We are talking about CLOCK_MONOTONIC. Steping is disabled on this clock.

Victor

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