[Python-Dev] PEP 418: rename time.monotonic() to time.steady()?

[Victor Stinner]

Hi,

I would to rename time.monotonic() to time.steady() in the PEP 418 for
the following reasons:

 - time.steady() may fallback to the system clock which is not
monotonic, it's strange to have to check for
time.get_clock_info('monotonic')['is_monotonic']
 - time.steady() uses GetTickCount() instead of
QueryPerformanceCounter() whereas both are monotonic, but
QueryPerformanceCounter() is not steady

Python steady clock will be different than the C++ definition.

You may argue that time.steady() is not always steady: it may fallback
to the system clock which is adjusted by NTP and can jump
backward/forward with a delta greater than 1 hour. In practice, there
is only one operating system that does not provide a monotonic clock:
GNU/Hurd.

I hesitate to add "is_steady" to time.get_clock_info(), but a boolean
is not very useful, it would be better to have a number.

Arguments for time.monotonic() name:

 - Users are looking for the "monotonic" name
 - Most of the time, time.monotonic() is a monotonic clock

--

On Linux, we might use CLOCK_MONOTONIC for time.steady() and
CLOCK_MONOTONIC_RAW for time.highres(). The NTP daemon Linux on Linux
uses a reliable clock to adjust CLOCK_MONOTONIC frequency and so
CLOCK_MONOTONIC is steady and it may go backward in a short period,
whereas CLOCK_MONOTONIC_RAW cannot go backward and so may fit closer
time.highres() requirements.

Currently, CLOCK_MONOTONIC is used for time.highres() and
time.steady() in the PEP.

--

NTP on Linux should only slew CLOCK_MONOTONIC, not step it. But it
looks like there was a bug in the Linux kernel 2.6.31: CLOCK_MONOTONIC
goes backward sometimes. Bug introduced in 2.6.31 by (August 14,
2009):
https://github.com/torvalds/linux/commit/0a54419836254a27baecd9037103171bcbabaf67
and fixed in the kernel 2.6.32 by (November 16, 2009):
https://github.com/torvalds/linux/commit/0696b711e4be45fa104c12329f617beb29c03f78

Someone had the bug:
http://stackoverflow.com/questions/3657289/linux-clock-gettimeclock-monotonic-strange-non-monotonic-behavior

Victor
PS: I already changed time.monotonic() to time.steady() in the PEP :-p