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

Janzert janzert at janzert.com
Wed Apr 11 05:31:30 CEST 2012

On 4/7/2012 5:49 AM, 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

Right, the description in that paragraph is exactly what I was referring 
to above. :) It is unfortunate that a clock with a resolution of 1ns may 
be purposefully thrown off by 500,000ns per second in the short term.

In practice you are probably correct that it is better to take the 
slewed clock even though it may have purposeful short term inaccuracy 
thrown in than it is to use the completely unadjusted one.

>> So for benchmarking it would not be surprising to be better off with the
>> non-adjusted clock. Ideally there would be a clock that was slewed "just
>> enough" to try and achieve short term accuracy, but I don't know of anything
>> providing that.
> time.monotonic() is not written for benchmarks. It does not have the
> highest frequecency, it's primary property is that is monotonic. A
> side effect is that it is usually the steadiest clock.
> For example, on Windows time.monotonic() has only an accuracy of 15 ms
> (15 milliseconds not 15 microseconds).

Hmm, I just realized an unfortunate result of that. Since it means 
time.monotonic() will be too coarse to be useful for frame rate level 
timing on windows.


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