[Python-Dev] PEP 418: Add monotonic clock

[Matt Joiner]

On Mar 28, 2012 8:38 AM, "Victor Stinner" <victor.stinner at gmail.com> wrote:
>
> Scott wrote:
>
> << The Boost implementation can be summarized as:
>
> system_clock:
>
>  mac = gettimeofday
>  posix = clock_gettime(CLOCK_REALTIME)
>  win = GetSystemTimeAsFileTime
>
> steady_clock:
>
>  mac = mach_absolute_time
>  posix = clock_gettime(CLOCK_MONOTONIC)
>  win = QueryPerformanceCounter
>
> high_resolution_clock:
>
>  * = { steady_clock, if available
>       system_clock, otherwise } >>
>
> I read again the doc of the QElapsedTimer class of the Qt library. So Qt
and Boost agree to say that QueryPerformanceCounter() *is* monotonic.
>
> I was confused because of a bug found in 2006 in Windows XP on multicore
processors. QueryPerformanceCounter() gave a different value on each core.
The bug was fixed in Windows and is known as KB896256 (I already added a
link to the bug in the PEP).
>
>>> I added a time.hires() clock to the PEP for the benchmarking/profiling
>>> use case (...)
>>
>>
>> It is this always-having-to-manually-fallback-depending-on-os that I was
>> hoping your new functionality would avoid. Is time.try_monotonic()
>> suitable for this usecase?
>
>
> If QueryPerformanceCounter() is monotonic, the API can be simplified to:
>
>  * time.time() = system clock
>  * time.monotonic() = monotonic clock
>  * time.hires() = monotonic clock or fallback to system clock
>
> time.hires() definition is exactly what I was trying to implement with
"time.steady(strict=True)" / "time.try_monotonic()".
>
> --
>
> Scott> monotonic_clock = always goes forward but can be adjusted
> Scott> steady_clock = always goes forward and cannot be adjusted
>
> I don't know if the monotonic clock should be called time.monotonic() or
time.steady(). The clock speed can be adjusted by NTP, at least on Linux <
2.6.28.

Monotonic. It's still monotonic if it is adjusted forward, and that's okay.

>
> I don't know if other clocks used by my time.monotonic() proposition can
be adjusted or not.
>
> If I understand correctly, time.steady() cannot be implemented using
CLOCK_MONOTONIC on Linux because CLOCK_MONOTONIC can be adjusted?
>
> Does it really matter if a monotonic speed is adjusted?
>
>
> Victor
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