[Python-Dev] Questions for the PEP 418: monotonic vs steady, is_adjusted (original) (raw)

Victor Stinner victor.stinner at gmail.com
Sat Apr 14 02:51:09 CEST 2012

Hi,

Before posting a first draft of the PEP 418 to python-dev, I have some questions.

== Naming: time.monotonic() or time.steady()? ==

I like the "steady" name but different people complained that the steady name should not be used if the function falls back to the system clock or if the clock is adjusted.

time.monotonic() does not fallback to the system clock anymore, it is now always monotonic.

There is only one clock used by time.monotonic() which is adjusted: CLOCK_MONOTONIC on Linux. On Linux, CLOCK_MONOTONIC is slewed by NTP, but not stepped. From the user point of view, the clock is steady. IMO CLOCK_MONOTONIC_RAW is less steady than CLOCK_MONOTONIC. CLOCK_MONOTONIC_RAW does drift from the real time, whereas NTP adjusts CLOCK_MONOTONIC to make it following closer to the real time. (I mean "real time" as defined in the Glossary of the PEP, not "civil time.)

I prefer "steady" over "monotonic" because the steady property is what users really expect from a "monotonic" clock. A monotonic but not steady clock may be useless.

All clocks used by the time.monotonic() of the PEP are steady. time.monotonic() should be the most steady clock of all available clocks. It may not have the best precision, use time.perf_counter() is you need the highest available precision, but you don't care if the clock is steady or not.

== "is_adjusted" key of time.get_clock_info() ==

time.get_clock_info() returns a dict with an optional key: "is_adjusted". This flag indicates "if the clock can be adjusted".

Should it be called "is_adjustable" instead? On Windows, the flag value may change at runtime when NTP is enabled or disabled. So the value is the current status of the clock adjustement. The description may be changed to "if the clock is adjusted".

Is a single flag enough? Or would be it better to indicate if the clock: only slewed, slewed and stepped, or not adjusted? (3 possible values) I guess that a single flag is enough. If you need more precise information, use the "implementation" information.

Victor

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