[Python-Dev] PEP 418: Add monotonic clock

Wed Mar 28 02:36:18 CEST 2012

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