python-peps/pep-0418.txt

PEP: 418
Title: Add monotonic and high-resolution time functions
Version: $Revision$
Last-Modified: $Date$
Author: Victor Stinner <victor.stinner@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-March-2012
Python-Version: 3.3


Abstract
========

Add time.monotonic() and time.highres() functions to Python 3.3.


Rationale
=========

Use cases:

 * Display the current time to a human (e.g. display a calendar or draw a wall
   clock): use system clock. time.time() or datetime.datetime.now()
 * Benchmark, profiling, timeout: time.highres()
 * Event scheduler: time.monotonic()


Functions
=========

time.time()
-----------

The system time is the "wall clock". It can be set manually by the system
administrator or automatically by a NTP daemon.  It can jump backward and
forward. It is not monotonic.

It is available on all platforms and cannot fail.

Pseudo-code [#pseudo]_: ::

    if os.name == "nt":
        def time():
            return _time.GetSystemTimeAsFileTime()
    else:
        def time():
            if hasattr(time, "clock_gettime"):
                try:
                    # resolution = 1 nanosecond
                    return time.clock_gettime(time.CLOCK_REALTIME)
                except OSError:
                    # CLOCK_REALTIME is not supported (unlikely)
                    pass
            if hasattr(_time, "gettimeofday"):
                try:
                    # resolution = 1 microsecond
                    return _time.gettimeofday()
                except OSError:
                    # gettimeofday() should not fail
                    pass
            if hasattr(_time, "ftime"):
                # resolution = 1 millisecond
                return _time.ftime()
            else:
                # resolution = 1 second
                return _time.time()


time.monotonic()
----------------

Clock advancing at a monotonic rate relative to real time. It cannot go
backward. Its rate may be adjusted by NTP. The reference point of the returned
value is undefined so only the difference of consecutive calls is valid.

It is not available on all platforms and may raise an OSError. It is not
available on GNU/Hurd for example.

The monotonic clock may stop while the system is suspended.

Pseudo-code [#pseudo]_: ::

    if os.name == 'nt':
        # GetTickCount64() requires Windows Vista, Server 2008 or later
        if hasattr(time, '_GetTickCount64'):
            _get_tick_count = _time.GetTickCount64
        else:
            def _get_tick_count():
                ticks = _time.GetTickCount()
                if ticks < _get_tick_count.last:
                    # Integer overflow detected
                    _get_tick_count.delta += 2**32
                _get_tick_count.last = ticks
                return ticks + _get_tick_count.delta
            _get_tick_count.last = 0
            _get_tick_count.delta = 0

        def monotonic():
            if monotonic.use_performance_counter:
                try:
                    return _time.QueryPerformanceCounter()
                except OSError:
                    # QueryPerformanceFrequency() may fail, if the installed
                    # hardware does not support a high-resolution performance
                    # counter for example
                    monotonic.use_performance_counter = False
            # Fallback to GetTickCount/GetTickCount64 which has
            # a lower resolution
            return _get_tick_count()
        monotonic.use_performance_counter = True

    elif os.name == 'mac':
        def monotonic():
            if monotonic.factor is None:
                factor = _time.mach_timebase_info()
                monotonic.factor = timebase[0] / timebase[1]
            return _time.mach_absolute_time() * monotonic.factor
        monotonic.factor = None

    elif os.name.startswith('sunos'):
        def monotonic():
            if monotonic.use_clock_highres:
                try:
                    time.clock_gettime(time.CLOCK_HIGHRES)
                except OSError:
                    monotonic.use_clock_highres = False
            return time.gethrtime()
        monotonic.use_clock_highres = (hasattr(time, 'clock_gettime')
                                       and hasattr(time, 'CLOCK_HIGHRES'))

    elif hasattr(time, "clock_gettime"):
        def monotonic():
            while monotonic.clocks:
                try:
                    clk_id = monotonic.clocks[0]
                    return time.clock_gettime(clk_id)
                except OSError:
                    # CLOCK_MONOTONIC_RAW requires a Linux kernel >= 2.6.28
                    del monotonic.clocks[0]
            return time.clock_gettime(time.CLOCK_MONOTONIC)
        monotonic.clocks = []
        if hasattr(time, 'CLOCK_MONOTONIC_RAW'):
            monotonic.clocks.append(time.CLOCK_MONOTONIC_RAW)
        if hasattr(time, 'CLOCK_HIGHRES'):
            monotonic.clocks.append(time.CLOCK_HIGHRES)

.. note::

   time.monotonic() detects GetTickCount() integer overflow (32 bits, roll-over
   after 49.7 days): it increases a delta by 2\ :sup:`32` each time than an
   overflow is detected. The delta is stored in the process local state and so
   time.monotonic() value may be different in two Python processes.


time.highres()
--------------

High-resolution clock: use a monotonic clock if available, or fallback to the
system time.

It is available on all platforms and cannot fail.

Pseudo-code::

    def highres():
        if highres.use_monotonic:
            try:
                return time.monotonic()
            except OSError:
                highres.use_monotonic = False
        return time.time()
    highres.use_monotonic = hasattr(time, 'monotonic')


Clocks
======

Monotonic
---------

mach_absolute_time
^^^^^^^^^^^^^^^^^^

Mac OS X provides a monotonic clock: mach_absolute_time(). It is based on
absolute elapsed time delta since system boot. It is not adjusted and cannot be
set.

mach_timebase_info() gives a fraction to convert the clock value to a number of
nanoseconds. According to the documentation (`Technical Q&A QA1398
<https://developer.apple.com/library/mac/#qa/qa1398/>`_), mach_timebase_info()
is always equals to one and does never fail, even if the function may fail
according to its prototype.

mach_absolute_time() has a resolution of 1 nanosecond.

CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW represents monotonic time since some
unspecified starting point. They cannot be set.

CLOCK_MONOTONIC is available at least on the following operating systems:

 * DragonFly BSD, FreeBSD >= 5.0, OpenBSD, NetBSD
 * Linux
 * Solaris

The following operating systems don't support CLOCK_MONOTONIC:


 * GNU/Hurd (see `open issues/ clock_gettime <http://www.gnu.org/software/hurd/open_issues/clock_gettime.html>`_)
 * Mac OS X
 * Windows

CLOCK_MONOTONIC_RAW is specific to Linux. It is similar to CLOCK_MONOTONIC, but
provides access to a raw hardware-based time that is not subject to NTP
adjustments. CLOCK_MONOTONIC_RAW requires Linux 2.6.28 or later.

On Linux, NTP may adjust CLOCK_MONOTONIC rate, but not jump backward.  If
available, CLOCK_MONOTONIC_RAW should be used instead of CLOCK_MONOTONIC to
avoid the NTP adjustement.

CLOCK_MONOTONIC stops while the machine is suspended.

clock_gettime() fails if the system does not support the specified clock,
whereas the standard C library supports it. For example, CLOCK_MONOTONIC_RAW
requires a kernel version 2.6.28 or later.

clock_getres() gives the clock resolution. It is 1 nanosecond on Linux.

.. note::
   clock_gettime() requires to link the program to the rt (real-time) library.

Windows: QueryPerformanceCounter
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

High-resolution performance counter. It is monotonic.
QueryPerformanceFrequency() gives its frequency.

On Windows XP, QueryPerformanceFrequency() is the processor frequency and
QueryPerformanceCounter() is the TSC of the current processor.

The performance counter value may unexpectedly leap forward because of a
hardware bug, see the `KB274323`_.

Windows XP had a bug (see `KB896256`_): on a multiprocessor computer,
QueryPerformanceCounter() returned a different value for each processor.

QueryPerformanceFrequency() should only be called once:  the frequency will not
change while the system is running. It fails if the installed hardware does not
support a high-resolution performance counter.

.. _KB896256: http://support.microsoft.com/?id=896256
.. _KB274323: http://support.microsoft.com/?id=274323


Windows: GetTickCount(), GetTickCount64()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

GetTickCount() and GetTickCount64() are monotonic and cannot fail.

GetTickCount64() was added to Windows Vista and Windows Server 2008.

The clock resolution is 1 millisecond. Its accuracy is usually around 15 ms. It
is possible to improve the accuracy using the `undocumented
NtSetTimerResolution() function
<http://undocumented.ntinternals.net/UserMode/Undocumented%20Functions/Time/NtSetTimerResolution.html>`_.
There are applications using this undocumented function, example:
`Timer Resolution <http://www.lucashale.com/timer-resolution/>`_.


Solaris: CLOCK_HIGHRES
^^^^^^^^^^^^^^^^^^^^^^

The Solaris OS has an CLOCK_HIGHRES timer that attempts to use an optimal
hardware source, and may give close to nanosecond resolution. CLOCK_HIGHRES is
the nonadjustable, high-resolution clock. For timers created with a clockid_t
value of CLOCK_HIGHRES, the system will attempt to use an optimal hardware
source.

Solaris: gethrtime
^^^^^^^^^^^^^^^^^^

The gethrtime() function returns the current high-resolution real time. Time is
expressed as nanoseconds since some arbitrary time in the past; it is not
correlated in any  way  to the  time  of  day,  and thus is not subject to
resetting or drifting by way of adjtime() or settimeofday(). The  hires timer
is  ideally  suited  to  performance measurement tasks, where cheap, accurate
interval timing is required.

On Solaris, gethrtime() is the same as clock_gettime(CLOCK_MONOTONIC).


System time
-----------

Windows: GetSystemTimeAsFileTime
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The system time can be read using GetSystemTimeAsFileTime(), ftime() and
time().

The system time resolution can be read using GetSystemTimeAdjustment(). The
accurary is usually between 0.5 millisecond and 15 milliseconds. Resolution:

 * GetSystemTimeAsFileTime(): 100 nanoseconds
 * ftime(): 1 millisecond
 * time(): 1 second

The system time can be set using SetSystemTime().

System time on UNIX
^^^^^^^^^^^^^^^^^^^

gettimeofday(), ftime(), time() and clock_gettime(CLOCK_REALTIME) return the
system clock.

Resolution:

 * clock_gettime(): clock_getres(CLOCK_REALTIME), 1 nanosecond on Linux
 * gettimeofday(): 1 microsecond
 * ftime(): 1 millisecond
 * time(): 1 second

The system time can be set using settimeofday() or clock_settime(CLOCK_REALTIME).


Process and thread time
-----------------------

The process and thread time cannot be set. They are not monotonic: the clocks
stop while the process/thread is idle.

Process
^^^^^^^

 * Windows: GetProcessTimes()
 * clock_gettime(CLOCK_PROCESS_CPUTIME_ID): High-resolution per-process timer
   from the CPU.
 * clock():

   * Windows:  The elapsed wall-clock time since the start of the process
     (elapsed time in seconds times CLOCKS_PER_SEC). It can fail.
   * UNIX: returns an approximation of processor time used by the program.

 * times()
 * getrusage(): ru_utime and ru_stime fields

Resolution:

 * clock() rate is CLOCKS_PER_SEC. It was called CLK_TCK in Microsoft C before
   6.0. On Linux 3, clock() has a resolution of 1 microsecond
 * The clock resolution can be read using clock_getres().
   clock_getres(CLOCK_REALTIME) is 1 nanosecond on Linux
 * GetProcessTimes(): call GetSystemTimeAdjustment()

Thread
^^^^^^

 * Windows: GetThreadTimes()
 * clock_gettime(CLOCK_THREAD_CPUTIME_ID): Thread-specific CPU-time clock.

Resolution:

 * CLOCK_THREAD_CPUTIME_ID: call clock_getres(). 1 nanosecond on Linux.
 * GetThreadTimes(): call GetSystemTimeAdjustment()

See also pthread_getcpuclockid().


Windows: QueryUnbiasedInterruptTime
-----------------------------------

Gets the current unbiased interrupt time from the biased interrupt time and the
current sleep bias amount. This time is not affected by power management sleep
transitions.

Is it monotonic?

QueryUnbiasedInterruptTime() was introduced in Windows 7.


Alternatives: API design
========================

time.highres() function name
----------------------------

Other names were proposed:

 * time.hires(): "hires" can be read as "to hire" as in "he hires a car to go
   on holiday", rather than a "HIgh-RESolution clock".
 * time.steady(): no OS provides a clock advancing at a steady rate, so
   "steady" should be avoided.
 * time.try_monotonic(): it is a clear and obvious solution for the use-case of
   "I prefer the monotonic clock, if it is available, otherwise I'll take my
   chances with a best-effect clock."
 * time.wallclock()

One function with a flag: time.monotonic(strict=False)
------------------------------------------------------

 * time.monotonic(strict=False) falls back to the system clock if no monotonic
   clock is available or if the monotonic clock failed.
 * time.monotonic(strict=True) raises OSError if monotonic clock fails and
   NotImplementedError if the system does not provide a monotonic clock

"A keyword argument that gets passed as a constant in the caller is usually
poor API."

Raising NotImplementedError for a function is something uncommon in Python and
should be avoided.


One function, no flag
---------------------

time.monotonic() returns (time: float, is_monotonic: bool).

An alternative is to use a function attribute: time.monotonic.is_monotonic. The
attribute value would be None before the first call to time.monotonic().


Working around operating system bugs?
=====================================

Should Python ensure manually that a monotonic clock is truly monotonic by
computing the maximum with the clock value and the previous value?

Since it's relatively straightforward to cache the last value returned using a
static variable, it might be interesting to use this to make sure that the
values returned are indeed monotonic.

 * Virtual machines provide less reliable clocks.
 * QueryPerformanceCounter() has two known bugs:
   `KB896256`_ (fixed) and `KB274323`_

Python may only workaround a specific known operating system bug: `KB274323`_
contains a code example to workaround the bug (use GetTickCount() to detect
QueryPerformanceCounter() leap).


Footnotes
=========

.. [#pseudo] "_time" is an hypothetical module only used for the example.
   The time module is implemented in C and so there is no need for such module.


Links
=====

Related Python issues:

 * `Issue #12822: NewGIL should use CLOCK_MONOTONIC if possible.
   <http://bugs.python.org/issue12822>`_
 * `Issue #14222: Use time.steady() to implement timeout
   <http://bugs.python.org/issue14222>`_
 * `Issue #14397: Use GetTickCount/GetTickCount64 instead of QueryPerformanceCounter for monotonic clock
   <http://bugs.python.org/issue14397>`_
 * `Issue #14428: Implementation of the PEP 418
   <http://bugs.python.org/issue14428>`_

Librairies exposing monotonic clocks:

 * `Qt library: QElapsedTimer
   <http://qt-project.org/doc/qt-4.8/qelapsedtimer.html>`_
 * `glib library: g_get_monotonic_time ()
   <http://developer.gnome.org/glib/2.30/glib-Date-and-Time-Functions.html>`_
   uses GetTickCount64()/GetTickCount() on Windows,
   clock_gettime(CLOCK_MONOTONIC) on UNIX or falls back to the system clock
 * `python-monotonic-time
   <http://code.google.com/p/python-monotonic-time/>`_
   (`github <https://github.com/gavinbeatty/python-monotonic-time>`_)
 * `monotonic_clock
   <https://github.com/ThomasHabets/monotonic_clock>`_
 * `Perl: Time::HiRes
   <http://perldoc.perl.org/Time/HiRes.html>`_ exposes
   clock_gettime(CLOCK_MONOTONIC)

Related documents:

 * `C++ Timeout Specification
   <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3128.html>`_
 * `Windows: Game Timing and Multicore Processors
   <http://msdn.microsoft.com/en-us/library/ee417693.aspx>`_
 * `Implement a Continuously Updating, High-Resolution Time Provider for Windows
   <http://msdn.microsoft.com/en-us/magazine/cc163996.aspx>`_