[Python-Dev] PEP 418: Add monotonic clock

[Victor Stinner]

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.

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