[Python-Dev] PEP 418: Add monotonic clock

[Zooko Wilcox-O'Hearn]

>  system_clock = wall clock time
>  monotonic_clock = always goes forward but can be adjusted
>  steady_clock = always goes forward and cannot be adjusted
>  high_resolution_clock = steady_clock || system_clock

Note that the C++ standard deprecated monotonic_clock once they
realized that there is absolutely no point in having a clock that
jumps forward but not back, and that none of the operating systems
implement such a thing -- instead they all implement a clock which
doesn't jump in either direction.

http://stackoverflow.com/questions/6777278/what-is-the-rationale-for-renaming-monotonic-clock-to-steady-clock-in-chrono

In other words, yes! +1! The C++ standards folks just went through the
process that we're now going through, and if we do it right we'll end
up at the same place they are:

http://en.cppreference.com/w/cpp/chrono/system_clock

"""
system_clock represents the system-wide real time wall clock. It may
not be monotonic: on most systems, the system time can be adjusted at
any moment. It is the only clock that has the ability to map its time
points to C time, and, therefore, to be displayed.

steady_clock: monotonic clock that will never be adjusted

high_resolution_clock: the clock with the shortest tick period available
"""

Note that we don't really have the option of providing a clock which
is "monotonic but not steady" in the sense of "can jump forward but
not back". It is a misunderstanding (doubtless due to the confusing
name "monotonic") to think that such a thing is offered by the
underlying platforms. We can choose to *call* it "monotonic",
following POSIX instead of calling it "steady", following C++.

Regards,

Zooko