python-peps/pep-0418.txt

PEP: 418
Title: Add monotonic time, performance counter and process time functions
Version: $Revision$
Last-Modified: $Date$
Author: Jim Jewett <jimjjewett@gmail.com>, 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
========

This PEP proposes to add ``time.get_clock_info(name)``,
``time.monotonic()``, ``time.perf_counter()`` and
``time.process_time()`` functions to Python 3.3.


Rationale
=========

If a program uses the system clock to schedule events or to implement
a timeout, it will not run events at the right moment or stop the
timeout too early or too late when the system clock is set manually or
adjusted automatically by NTP.  A monotonic clock should be used
instead to not be affected by system clock updates.

To measure the performance of a function, ``time.clock()`` can be used
but it is very different on Windows and on Unix.  On Windows,
``time.clock()`` includes time elapsed during sleep, whereas it does
not on Unix.  ``time.clock()`` precision is very good on Windows, but
very bad on Unix.  The new ``time.perf_counter()`` function can be
used instead to always get the most precise performance counter with a
portable behaviour.

To measure CPU time, Python does not provide directly a portable
function.  ``time.clock()`` can be used on Unix, but it has a bad
precision.  ``resource.getrusage()`` can also be used on Unix, but it
requires to get fields of a structure and compute the sum of time
spent in kernel space and user space.  The new ``time.process_time()``
function acts as a portable counter that always measures CPU time
(doesn't include time elapsed during sleep) and has the best available
precision.

Each operating system implements clocks and performance counters
differently, and it is useful to know exactly which function is used
and some properties of the clock like its resolution and its
precision.  The new ``time.get_clock_info()`` function gives access to
all available information of each Python time function.

New functions:

* ``time.monotonic()``: timeout and scheduling, not affected by system
  clock updates
* ``time.perf_counter()``: benchmarking, most precise clock for short
  period
* ``time.process_time()``: profiling, CPU time of the process

Users of new functions:

 * time.monotonic(): concurrent.futures, multiprocessing, queue, subprocess,
   telnet and threading modules to implement timeout
 * time.perf_counter(): trace and timeit modules, pybench program
 * time.process_time(): profile module
 * time.get_clock_info(): pybench program to display information about the
   timer like the precision or the resolution

The ``time.clock()`` function is deprecated by this PEP because it is not
portable: it behaves differently depending on the operating system.
``time.perf_counter()`` or ``time.process_time()`` should be used instead,
depending on your requirements. ``time.clock()`` is marked as deprecated but
is not planned for removal.


Python functions
================

New functions
-------------

time.get_clock_info(name)
^^^^^^^^^^^^^^^^^^^^^^^^^

Get information on the specified clock.  Supported clocks:

* "clock": time.clock()
* "monotonic": time.monotonic()
* "perf_counter": time.perf_counter()
* "process_time": time.process_time()
* "time": time.time()

Return a dictionary with the following keys:

* Mandatory keys:

  * "implementation" (str): name of the underlying operating system
    function.  Examples: "QueryPerformanceCounter()",
    "clock_gettime(CLOCK_REALTIME)".
  * "resolution" (float): resolution in seconds of the clock
  * "is_monotonic" (bool): True if the clock cannot go backward

* Optional keys:

  * "precision" (float): precision in seconds of the clock
  * "is_adjusted" (bool): True if the clock can be adjusted (e.g. by a
    NTP daemon)


time.monotonic()
^^^^^^^^^^^^^^^^

Monotonic clock, cannot go backward.  It is not affected by system
clock updates.  The reference point of the returned value is
undefined, so that only the difference between the results of
consecutive calls is valid.

Availability: Windows, Mac OS X, Unix, Solaris. Not available on GNU/Hurd.

Pseudo-code [#pseudo]_::

    if os.name == 'nt':
        # GetTickCount64() requires Windows Vista, Server 2008 or later
        if hasattr(time, '_GetTickCount64'):
            def monotonic():
                return _time.GetTickCount64() * 1e-3
        else:
            def monotonic():
                ticks = _time.GetTickCount()
                if ticks < monotonic.last:
                    # Integer overflow detected
                    monotonic.delta += 2**32
                monotonic.last = ticks
                return (ticks + monotonic.delta) * 1e-3
            monotonic.last = 0
            monotonic.delta = 0

    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 hasattr(time, "clock_gettime"):
        def monotonic():
            if monotonic.use_clock_highres:
                try:
                    time.clock_gettime(time.CLOCK_HIGHRES)
                except OSError:
                    monotonic.use_clock_highres = False
            return time.clock_gettime(time.CLOCK_MONOTONIC)
        monotonic.use_clock_highres = (hasattr(time, 'clock_gettime')
                                       and hasattr(time, 'CLOCK_HIGHRES'))


On Windows, ``QueryPerformanceCounter()`` is not used even though it
has a better precision than ``GetTickCount()``.  It is not reliable
and has too many issues.

``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 the value of ``time.monotonic()`` may be different in two
Python processes running for more than 49 days.


time.perf_counter()
^^^^^^^^^^^^^^^^^^^

Performance counter used for benchmarking.  It is monotonic,
i.e. cannot go backward.  It does include time elapsed during sleep.
The reference point of the returned value is undefined, so that only
the difference between the results of consecutive calls is valid and
is number of seconds.

Pseudo-code::

    def perf_counter():
        if perf_counter.use_performance_counter:
            if perf_counter.perf_frequency is None:
                try:
                    perf_counter.perf_frequency = float(_time.QueryPerformanceFrequency())
                except OSError:
                    # QueryPerformanceFrequency() fails if the installed
                    # hardware does not support a high-resolution performance
                    # counter
                    perf_counter.use_performance_counter = False
                else:
                    return _time.QueryPerformanceCounter() / perf_counter.perf_frequency
            else:
                return _time.QueryPerformanceCounter() / perf_counter.perf_frequency
        if perf_counter.use_monotonic:
            # Monotonic clock is preferred over system clock
            try:
                return time.monotonic()
            except OSError:
                perf_counter.use_monotonic = False
        return time.time()
    perf_counter.use_performance_counter = (os.name == 'nt')
    if perf_counter.use_performance_counter:
        perf_counter.perf_frequency = None
    perf_counter.use_monotonic = hasattr(time, 'monotonic')


time.process_time()
^^^^^^^^^^^^^^^^^^^

Process time used for profiling: sum of the kernel and user-space CPU
time.  It does not include time elapsed during sleep.  The reference
point of the returned value is undefined, so that only the difference
between the results of consecutive calls is valid.

Pseudo-code [#pseudo]_::

    if os.name == 'nt':
        def process_time():
            handle = win32process.GetCurrentProcess()
            process_times = win32process.GetProcessTimes(handle)
            return (process_times['UserTime'] + process_times['KernelTime']) * 1e-7
    else:
        import os
        try:
            import resource
        except ImportError:
            has_resource = False
        else:
            has_resource = True

        def process_time():
            if process_time.use_process_cputime:
                try:
                    return time.clock_gettime(time.CLOCK_PROCESS_CPUTIME_ID)
                except OSError:
                    process_time.use_process_cputime = False
            if process_time.use_getrusage:
                try:
                    usage = resource.getrusage(resource.RUSAGE_SELF)
                    return usage[0] + usage[1]
                except OSError:
                    process_time.use_getrusage = False
            if process_time.use_times:
                try:
                    times = os.times()
                    return times[0] + times[1]
                except OSError:
                    process_time.use_getrusage = False
            return _time.clock()
        process_time.use_process_cputime = (
            hasattr(time, 'clock_gettime')
            and hasattr(time, 'CLOCK_PROCESS_CPUTIME_ID'))
        process_time.use_getrusage = has_resource
        # On OS/2, only the 5th field of os.times() is set, others are zeros
        process_time.use_times = (hasattr(os, 'times') and os.name != 'os2')


Existing 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:
                    return time.clock_gettime(time.CLOCK_REALTIME)
                except OSError:
                    # CLOCK_REALTIME is not supported (unlikely)
                    pass
            if hasattr(_time, "gettimeofday"):
                try:
                    return _time.gettimeofday()
                except OSError:
                    # gettimeofday() should not fail
                    pass
            if hasattr(_time, "ftime"):
                return _time.ftime()
            else:
                return _time.time()


time.sleep()
^^^^^^^^^^^^

Suspend execution for the given number of seconds.  The actual
suspension time may be less than that requested because any caught
signal will terminate the ``time.sleep()`` following execution of that
signal's catching routine.  Also, the suspension time may be longer
than requested by an arbitrary amount because of the scheduling of
other activity in the system.

Pseudo-code [#pseudo]_::

    try:
        import select
    except ImportError:
        has_select = False
    else:
        has_select = hasattr(select, "select")

    if has_select:
        def sleep(seconds):
            return select.select([], [], [], seconds)

    elif hasattr(_time, "delay"):
        def sleep(seconds):
            milliseconds = int(seconds * 1000)
            _time.delay(milliseconds)

    elif os.name == "nt":
        def sleep(seconds):
            milliseconds = int(seconds * 1000)
            win32api.ResetEvent(hInterruptEvent);
            win32api.WaitForSingleObject(sleep.sigint_event, milliseconds)

        sleep.sigint_event = win32api.CreateEvent(NULL, TRUE, FALSE, FALSE)
        # SetEvent(sleep.sigint_event) will be called by the signal handler of SIGINT

    elif os.name == "os2":
        def sleep(seconds):
            milliseconds = int(seconds * 1000)
            DosSleep(milliseconds)

    else:
        def sleep(seconds):
            seconds = int(seconds)
            _time.sleep(seconds)

Deprecated functions
--------------------

time.clock()
^^^^^^^^^^^^

On Unix, return the current processor time as a floating point number
expressed in seconds.  The precision, and in fact the very definition
of the meaning of "processor time", depends on that of the C function
of the same name, but in any case, this is the function to use for
benchmarking Python or timing algorithms.

On Windows, this function returns wall-clock seconds elapsed since the
first call to this function, as a floating point number, based on the
Win32 function ``QueryPerformanceCounter()``.  The resolution is
typically better than one microsecond.

Pseudo-code [#pseudo]_::

    if os.name == 'nt':
        def clock():
            if clock.use_performance_counter:
                if clock.perf_frequency is None:
                    try:
                        clock.perf_frequency = float(_time.QueryPerformanceFrequency())
                    except OSError:
                        # QueryPerformanceFrequency() fails if the installed
                        # hardware does not support a high-resolution performance
                        # counter
                        clock.use_performance_counter = False
                    else:
                        return _time.QueryPerformanceCounter() / clock.perf_frequency
                else:
                    return _time.QueryPerformanceCounter() / clock.perf_frequency
            return _time.clock()
        clock.use_performance_counter = True
        clock.perf_frequency = None
    else:
        clock = _time.clock



Glossary
========

:Accuracy:
   Is the answer correct?  Any clock will eventually <drift>; if a
   clock is intended to match <Civil Time>, it will need to be
   <adjusted> back to the "true" time. See also `Accuracy and precision
   <http://en.wikipedia.org/wiki/Accuracy_and_precision>`_.

:Adjusted:
   Resetting a clock to the correct time.  This may be done either
   with a <Step> or by <Slewing>.

:Civil Time:
   Time of day; external to the system.  10:45:13am is a Civil time;
   45 seconds is not.  Provided by existing function
   ``time.localtime()`` and ``time.gmtime()``.  Not changed by this
   PEP.

:Clock:
   An instrument for measuring time.  Different clocks have different
   characteristics; for example, a clock with <nanonsecond>
   <precision> may start to <drift> after a few minutes, while a less
   precise clock remained accurate for days.  This PEP is primarily
   concerned with clocks which use a unit of seconds.

:Counter:
   A clock which increments each time a certain event occurs.  A
   counter is <strictly monotonic>, but not <clock_monotonic>.  It can
   be used to generate a unique (and ordered) timestamp, but these
   timestamps cannot be mapped to <civil time>; tick creation may well
   be bursty, with several advances in the same millisecond followed
   by several days without any advance.

:CPU Time:
   A measure of how much CPU effort has been spent on a certain task.
   CPU seconds are often normalized (so that a variable number can
   occur in the same actual second).  CPU seconds can be important
   when profiling, but they do not map directly to user response time,
   nor are they directly comparable to (real time) seconds.

:Duration:
   Elapsed time.  The difference between the starting and ending
   times.  A defined <epoch> creates an implicit (and usually large)
   duration.  More precision can generally be provided for a
   relatively small <duration>.

:Drift:
   The accumulated error against "true" time, as defined externally to
   the system.

:Epoch:
   The reference point of a clock.  For clocks providing <civil time>,
   this is often midnight as the day (and year) rolled over to January
   1, 1970.  For a <clock_monotonic> clock, the epoch may be undefined
   (represented as None).

:Latency:
   Delay.  By the time a clock call returns, the <real time> has
   advanced, possibly by more than the precision of the clock.

:Monotonic:
   The characteristics expected of a monotonic clock in practice.
   Moving in at most one direction; for clocks, that direction is
   forward. The <clock> should also be <steady>, and should be
   convertible to a unit of seconds.  The tradeoffs often include lack
   of a defined <epoch> or mapping to <Civil Time>, and being more
   expensive (in <latency>, power usage, or <duration> spent within
   calls to the clock itself) to use.  For example, the clock may
   represent (a constant multiplied by) ticks of a specific quartz
   timer on a specific CPU core, and calls would therefore require
   synchronization between cores.

:Precision:
   Significant Digits.  What is the smallest duration that the clock
   can distinguish?  This differs from <resolution> in that a
   difference greater than the minimum precision is actually
   meaningful.

:Process Time:
   Time elapsed since the process began.  It is typically measured in
   <CPU time> rather than <real time>, and typically does not advance
   while the process is suspended.

:Real Time:
   Time in the real world.  This differs from <Civil time> in that it
   is not <adjusted>, but they should otherwise advance in lockstep.
   It is not related to the "real time" of "Real Time [Operating]
   Systems".  It is sometimes called "wall clock time" to avoid that
   ambiguity; unfortunately, that introduces different ambiguities.

:Resolution:
   Represented Digits.  Note that many clocks will have a resolution
   greater than their actual <precision>.

:Slew:
   A slight change to a clock's speed, usually intended to correct
   <drift> with respect to an external authority.

:Stability:
   Persistence of accuracy.  A measure of expected <drift>.

:Steady:
   A clock with high <stability> and relatively high <accuracy> and
   <precision>.  In practice, it is often used to indicate a
   <clock_monotonic> clock, but places greater emphasis on the
   consistency of the duration between subsequent ticks.

:Step:
   An instantaneous change in the represented time.  Instead of
   speeding or slowing the clock (<slew>), a single offset is
   permanently added.

:System Time:
   Time as represented by the Operating System.

:Thread Time:
   Time elapsed since the thread began.  It is typically measured in
   <CPU time> rather than <real time>, and typically does not advance
   while the thread is idle.

:Wallclock:
   What the clock on the wall says.  This is typically used as a
   synonym for <real time>; unfortunately, wall time is itself
   ambiguous.


Hardware clocks
===============

List of hardware clocks
-----------------------

* HPET: An High Precision Event Timer (HPET) chip consists of a 64-bit
  up-counter (main counter) counting at least at 10 MHz and a set of
  up to 256 comparators (at least 3).  Each HPET can have up to 32
  timers.  HPET can cause around 3 seconds of drift per day.
* TSC (Time Stamp Counter): Historically, the TSC increased with every
  internal processor clock cycle, but now the rate is usually constant
  (even if the processor changes frequency) and usually equals the
  maximum processor frequency.  Multiple cores having different TSC
  values.  Hibernation of system will reset TSC value.  The RDTSC
  instruction can be used to read this counter.  CPU frequency scaling
  for power saving.
* ACPI Power Management Timer: ACPI 24-bit timer with a frequency of
  3.5 MHz (3,579,545 Hz).
* Cyclone: The Cyclone timer uses a 32-bit counter on IBM Extended
  X-Architecture (EXA) chipsets which include computers that use the
  IBM "Summit" series chipsets (ex: x440).  This is available in IA32
  and IA64 architectures.
* PIT (programmable interrupt timer): Intel 8253/8254 chipsets with a
  configurable frequency in range 18.2 Hz - 1.2 MHz.  It is a 16-bit
  counter.
* RTC (Real-time clock).  Most RTCs use a crystal oscillator with a
  frequency of 32,768 Hz


Linux clocksource
-----------------

There were 4 implementations of the time in the Linux kernel: UTIME
(1996), timer wheel (1997), HRT (2001) and hrtimers (2007).  The
latter is the result of the "high-res-timers" project started by
George Anzinger in 2001, with contributions by Thomas Gleixner and
Douglas Niehaus.  hrtimers implementation was merged into Linux
2.6.21, released in 2007.

hrtimers supports various clock sources.  It sets a priority to each
source to decide which one will be used. Linux supports the following
clock sources:

* tsc
* hpet
* pit
* pmtmr: ACPI Power Management Timer
* cyclone

High-resolution timers are not supported on all hardware
architectures.  They are at least provided on x86/x86_64, ARM and
PowerPC.

The ``/sys/devices/system/clocksource/clocksource0`` directory
contains two useful files:

* ``available_clocksource``: list of available clock sources
* ``current_clocksource``: clock source currently used.  It is
  possible to change the current clocksource by writing the name of a
  clocksource into this file.

``/proc/timer_list`` contains the list of all hardware timers.

Read also the `time(7) manual page
<http://www.kernel.org/doc/man-pages/online/pages/man7/time.7.html>`_:
"overview of time and timers".


FreeBSD timecounter
-------------------

kern.timecounter.choice list available hardware clocks with their
priority.  The sysctl program can be used to change the timecounter.
Example::

    # dmesg | grep Timecounter
    Timecounter "i8254" frequency 1193182 Hz quality 0
    Timecounter "ACPI-safe" frequency 3579545 Hz quality 850
    Timecounter "HPET" frequency 100000000 Hz quality 900
    Timecounter "TSC" frequency 3411154800 Hz quality 800
    Timecounters tick every 10.000 msec
    # sysctl kern.timecounter.choice
    kern.timecounter.choice: TSC(800) HPET(900) ACPI-safe(850) i8254(0) dummy(-1000000)
    # sysctl kern.timecounter.hardware="ACPI-fast"
    kern.timecounter.hardware: HPET -> ACPI-fast

Available clocks:

* "TSC": Time Stamp Counter of the processor
* "HPET": High Precision Event Timer
* "ACPI-fast": ACPI Power Management timer (fast mode)
* "ACPI-safe": ACPI Power Management timer (safe mode)
* "i8254": PIT with Intel 8254 chipset

The `commit 222222
<http://svnweb.freebsd.org/base?view=revision&revision=222222>`_ (May
2011) decreased ACPI-fast timecounter quality to 900 and increased
HPET timecounter quality to 950: "HPET on modern platforms usually
have better resolution and lower latency than ACPI timer".

Read `Timecounters: Efficient and precise timekeeping in SMP kernels
<http://phk.freebsd.dk/pubs/timecounter.pdf>`_ by Poul-Henning Kamp
(2002) for the FreeBSD Project.


Performance
-----------

Reading an hardware clock has a cost.  The following table compares
the performance of different hardware clocks on Linux 3.3 with Intel
Core i7-2600 at 3.40GHz (8 cores).

========================  ======  =======  ======
Function                  TSC     ACPI PM  HPET
========================  ======  =======  ======
time()                      2 ns     2 ns    2 ns
CLOCK_REALTIME_COARSE      10 ns    10 ns   10 ns
CLOCK_MONOTONIC_COARSE     12 ns    13 ns   12 ns
CLOCK_THREAD_CPUTIME_ID   134 ns   135 ns  135 ns
CLOCK_PROCESS_CPUTIME_ID  127 ns   129 ns  129 ns
clock()                   146 ns   146 ns  143 ns
gettimeofday()             23 ns   726 ns  637 ns
CLOCK_MONOTONIC_RAW        31 ns   716 ns  607 ns
CLOCK_REALTIME             27 ns   707 ns  629 ns
CLOCK_MONOTONIC            27 ns   723 ns  635 ns
========================  ======  =======  ======

Each function was called 10,000,000 times and CLOCK_MONOTONIC was used
to get the time before and after.  The benchmark was run 5 times to
keep the minimum time.

FreeBSD 8.0 in kvm with hardware virtualization:

========================  ======  =========  =======  =======
Function                  TSC     ACPI-Safe  HPET     i8254
========================  ======  =========  =======  =======
time()                    191 ns    188 ns    189 ns   188 ns
CLOCK_SECOND              187 ns    184 ns    187 ns   183 ns
CLOCK_REALTIME_FAST       189 ns    180 ns    187 ns   190 ns
CLOCK_UPTIME_FAST         191 ns    185 ns    186 ns   196 ns
CLOCK_MONOTONIC_FAST      188 ns    187 ns    188 ns   189 ns
CLOCK_THREAD_CPUTIME_ID   208 ns    206 ns    207 ns   220 ns
CLOCK_VIRTUAL             280 ns    279 ns    283 ns   296 ns
CLOCK_PROF                289 ns    280 ns    282 ns   286 ns
clock()                   342 ns    340 ns    337 ns   344 ns
CLOCK_UPTIME_PRECISE      197 ns  10380 ns   4402 ns  4097 ns
CLOCK_REALTIME            196 ns  10376 ns   4337 ns  4054 ns
CLOCK_MONOTONIC_PRECISE   198 ns  10493 ns   4413 ns  3958 ns
CLOCK_UPTIME              197 ns  10523 ns   4458 ns  4058 ns
gettimeofday()            202 ns  10524 ns   4186 ns  3962 ns
CLOCK_REALTIME_PRECISE    197 ns  10599 ns   4394 ns  4060 ns
CLOCK_MONOTONIC           201 ns  10766 ns   4498 ns  3943 ns
========================  ======  =========  =======  =======

Each function was called 100,000 times and CLOCK_MONOTONIC was used to
get the time before and after.  The benchmark was run 5 times to keep
the minimum time.


NTP adjustment
==============

NTP has different methods to adjust a clock:

* "slewing": change the clock frequency to be slightly faster or
  slower (which is done with ``adjtime()``).  Since the slew rate is
  limited to 0.5 millisecond per second, each second of adjustment requires an
  amortization interval of 2000 seconds.  Thus, an adjustment of many
  seconds can take hours or days to amortize.
* "stepping": jump by a large amount in a single discrete step (which
  is done with ``settimeofday()``)

By default, the time is slewed if the offset is less than 128 ms, or
stepped otherwise.

Slewing is generally desirable (i.e. we should use CLOCK_MONOTONIC,
not CLOCK_MONOTONIC_RAW) if one wishes to measure "real" time (and not
a time-like object like CPU cycles).  This is because the clock on the
other end of the NTP connection from you is probably better at keeping
time: hopefully that thirty five thousand dollars of Cesium
timekeeping goodness is doing something better than your PC's $3
quartz crystal, after all.

Get more detail in the `documentation of the NTP daemon
<http://doc.ntp.org/4.1.2/ntpd.htm>`_.


Operating system time functions
===============================

Monotonic clocks
----------------

=========================  ==========  ===============  =============  ===============
Name                       Resolution  Adjusted         Include sleep  Include suspend
=========================  ==========  ===============  =============  ===============
gethrtime()                1 ns        No               Yes            Yes
CLOCK_HIGHRES              1 ns        No               Yes            Yes
CLOCK_MONOTONIC            1 ns        Slewed on Linux  Yes            No
CLOCK_MONOTONIC_COARSE     1 ns        Slewed on Linux  Yes            No
CLOCK_MONOTONIC_RAW        1 ns        No               Yes            No
CLOCK_BOOTTIME             1 ns        ?                Yes            Yes
CLOCK_UPTIME               1 ns        No               Yes            ?
mach_absolute_time()       1 ns        No               Yes            No
QueryPerformanceCounter()  \-          No               Yes            ?
GetTickCount[64]()         1 ms        No               Yes            Yes
timeGetTime()              1 ms        No               Yes            ?
=========================  ==========  ===============  =============  ===============

Examples of clock precision on x86_64:

=========================  ================  =========  ===================
Name                       Operating system  Precision  Precision in Python
=========================  ================  =========  ===================
CLOCK_HIGHRES              SunOS 5.11             2 ns                 2 µs
CLOCK_MONOTONIC            Linux 3.2              1 ns                 2 µs
CLOCK_MONOTONIC            SunOS 5.11             2 ns                 3 µs
CLOCK_MONOTONIC_RAW        Linux 3.2              1 ns                 3 µs
CLOCK_BOOTTIME             Linux 3.3              1 ns                 3 µs
CLOCK_MONOTONIC            FreeBSD 8.2           11 ns                 7 µs
CLOCK_MONOTONIC            OpenBSD 5.0           10 ms                 7 µs
QueryPerformanceCounter    Windows Seven         10 ns                 9 µs
CLOCK_UPTIME               FreeBSD 8.2           11 ns                 9 µs
CLOCK_MONOTONIC_COARSE     Linux 3.3              1 ms                 1 ms
GetTickCount               Windows Seven         16 ms                15 ms
=========================  ================  =========  ===================

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 equal to one and never fails, even if
the function may fail according to its prototype.

mach_absolute_time() stops during a sleep on a PowerPC CPU, but not on
an Intel CPU: `Different behaviour of mach_absolute_time() on i386/ppc
<http://lists.apple.com/archives/PerfOptimization-dev/2006/Jul/msg00024.html>`_.

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

CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW represent monotonic time since
some unspecified starting point.  They cannot be set.  The precision
can be read using ``clock_getres()``.

Documentation: refer to the manual page of your operating system.
Examples:

* `FreeBSD clock_gettime() manual page
  <http://www.freebsd.org/cgi/man.cgi?query=clock_gettime>`_
* `Linux clock_gettime() manual page
  <http://linux.die.net/man/3/clock_gettime>`_

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

On Linux, NTP may adjust the CLOCK_MONOTONIC rate (slewed), but it cannot
jump backward.

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.

Linux 2.6.39 and glibc 2.14 introduces a new clock: CLOCK_BOOTTIME.
CLOCK_BOOTTIME is idential to CLOCK_MONOTONIC, except that it also
includes any time spent in suspend.  Read also `Waking systems from
suspend <http://lwn.net/Articles/429925/>`_ (March, 2011).

CLOCK_MONOTONIC stops while the machine is suspended.

Linux provides also CLOCK_MONOTONIC_COARSE since Linux 2.6.32. It is
similar to CLOCK_MONOTONIC, less precise but faster.

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


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

High-resolution performance counter.  It is monotonic.
The frequency of the counter can be read using QueryPerformanceFrequency().
The precision is 1 / QueryPerformanceFrequency().

It has a much higher resolution, but has lower long term precision
than GetTickCount() and timeGetTime() clocks.  For example, it will
drift compared to the low precision clocks.

Documentation:

* `MSDN: QueryPerformanceCounter() documentation
  <http://msdn.microsoft.com/en-us/library/windows/desktop/ms644904%28v=vs.85%29.aspx>`_
* `MSDN: QueryPerformanceFrequency() documentation
  <http://msdn.microsoft.com/en-us/library/windows/desktop/ms644905%28v=vs.85%29.aspx>`_

Hardware clocks used by QueryPerformanceCounter:

* Windows XP: RDTSC instruction of Intel processors, the clock
  frequency is the frequency of the processor (between 200 MHz and 3
  GHz, usually greater than 1 GHz nowadays).
* Windows 2000: ACPI power management timer, frequency = 3,549,545 Hz.
  It can be forced through the "/usepmtimer" flag in boot.ini.

.. * Windows 95/98: 8245 PIT chipset, frequency = 1,193,181 Hz

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.

QueryPerformanceCounter() cannot be adjusted:
`SetSystemTimeAdjustment()
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms724943(v=vs.85).aspx>`_
only adjusts the system time.

Bugs:

* The performance counter value may unexpectedly leap forward because
  of a hardware bug, see `KB274323`_.
* On VirtualBox, QueryPerformanceCounter() does not increment the high
  part every time the low part overflows, see `Monotonic timers
  <http://code-factor.blogspot.fr/2009/11/monotonic-timers.html>`_
  (2009).
* VirtualBox had a bug in its HPET virtualized device:
  QueryPerformanceCounter() did jump forward by approx. 42 seconds (`issue
  #8707 <https://www.virtualbox.org/ticket/8707>`_).
* Windows XP had a bug (see `KB896256`_): on a multiprocessor
  computer, QueryPerformanceCounter() returned a different value for
  each processor.  The bug was fixed in Windows XP SP2.
* Issues with processor with variable frequency: the frequency is
  changed depending on the workload to reduce memory consumption.
* Chromium don't use QueryPerformanceCounter() on Athlon X2 CPUs
  (model 15) because "QueryPerformanceCounter is unreliable" (see
  base/time_win.cc in Chromium source code)

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


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

GetTickCount() and GetTickCount64() are monotonic, cannot fail and are
not adjusted by SetSystemTimeAdjustment().  MSDN documentation:
`GetTickCount()
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms724408(v=vs.85).aspx>`_,
`GetTickCount64()
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms724411(v=vs.85).aspx>`_.
The precision can be read using GetSystemTimeAdjustment().

The elapsed time retrieved by GetTickCount() or GetTickCount64()
includes time the system spends in sleep or hibernation.

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

It is possible to improve the precision 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/>`_.

WaitForSingleObject() uses the same timer as GetTickCount() with the
same precision.


Windows: timeGetTime
^^^^^^^^^^^^^^^^^^^^

The timeGetTime function retrieves the system time, in milliseconds.
The system time is the time elapsed since Windows was started.  Read
the `timeGetTime() documentation
<http://msdn.microsoft.com/en-us/library/windows/desktop/dd757629(v=vs.85).aspx>`_.

The return type of timeGetTime() is a 32-bit unsigned integer.  As
GetTickCount(), timeGetTime() rolls over after 2^32 milliseconds (49.7
days).

The elapsed time retrieved by timeGetTime() includes time the system
spends in sleep.

The default precision of the timeGetTime function can be five
milliseconds or more, depending on the machine.

timeBeginPeriod() can be used to increase the precision of
timeGetTime() up to 1 millisecond, but it negatively affects power
consumption.  Calling timeBeginPeriod() also affects the granularity
of some other timing calls, such as CreateWaitableTimer(),
WaitForSingleObject() and Sleep().

.. note::

   timeGetTime() and timeBeginPeriod() are part the Windows multimedia
   library and so require to link the program against winmm or to
   dynamically load the library.

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

The Solaris OS has a 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.

The precision of CLOCK_HIGHRES can be read using ``clock_getres()``.

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.

The linearity of gethrtime() is not preserved across a suspend-resume
cycle (`Bug 4272663 <http://wesunsolve.net/bugid/id/4272663>`_).

Read the `gethrtime() manual page of Solaris 11
<http://docs.oracle.com/cd/E23824_01/html/821-1465/gethrtime-3c.html#scrolltoc>`_.

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


System time clocks
------------------

========================= ===============  =============  ===============
Name                      Resolution       Include sleep  Include suspend
========================= ===============  =============  ===============
CLOCK_REALTIME            1 ns             Yes            Yes
CLOCK_REALTIME_COARSE     1 ns             Yes            Yes
GetSystemTimeAsFileTime   100 ns           Yes            Yes
gettimeofday()            1 µs             Yes            Yes
ftime()                   1 ms             Yes            Yes
time()                    1 sec            Yes            Yes
========================= ===============  =============  ===============

Examples of clock precision on x86_64:

=========================  ================  =========  ===================
Name                       Operating system  Precision  Precision in Python
=========================  ================  =========  ===================
CLOCK_REALTIME             SunOS 5.11            10 ms                 2 µs
CLOCK_REALTIME             Linux 3.2              1 ns                 2 µs
CLOCK_REALTIME             FreeBSD 8.2           11 ns                 7 µs
CLOCK_REALTIME             OpenBSD 5.0           10 ms                10 µs
CLOCK_REALTIME_COARSE      Linux 3.3              1 ms                 1 ms
GetSystemTimeAsFileTime    Windows Seven         16 ms                 1 ms
=========================  ================  =========  ===================


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

The system time can be read using GetSystemTimeAsFileTime(), ftime() and
time(). The precision of the system clock can be read using
GetSystemTimeAdjustment().

Read the `GetSystemTimeAsFileTime() documentation
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms724397(v=vs.85).aspx>`_.

The system time can be set using SetSystemTime().

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

gettimeofday(), ftime(), time() and clock_gettime(CLOCK_REALTIME) return
the system clock. The precision of CLOCK_REALTIME can be read using
clock_getres().

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

Linux provides also CLOCK_REALTIME_COARSE since Linux 2.6.32. It is similar
to CLOCK_REALTIME, less precise but faster.

Alexander Shishkin proposed an API for Linux to be notified when the system
clock is changed: `timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes
<http://lwn.net/Articles/432395/>`_ (4th version of the API, March 2011). The
API is not accepted yet, CLOCK_BOOTTIME provides a similar feature.


Process time
------------

The process time cannot be set.  It is not monotonic: the clocks stop
while the process is idle.

=========================  ==========  =============  ===============
Name                       Resolution  Include sleep  Include suspend
=========================  ==========  =============  ===============
GetProcessTimes()              100 ns  No             No
CLOCK_PROCESS_CPUTIME_ID         1 ns  No             No
getrusage(RUSAGE_SELF)           1 µs  No             No
times()                            \-  No             No
clock()                            \-  No (*)         No
=========================  ==========  =============  ===============

(*) On Windows, clock() does include time elapsed during a sleep. It does not
on other operating systems.

Examples of clock precision on x86_64:

=========================  ================  =========  ===================
Name                       Operating system  Precision  Precision in Python
=========================  ================  =========  ===================
getrusage(RUSAGE_SELF)     FreeBSD 8.2              \-                 1 µs
CLOCK_PROCESS_CPUTIME_ID   Linux 3.2              1 ns                 3 µs
getrusage(RUSAGE_SELF)     SunOS 5.11               \-                 3 µs
getrusage(RUSAGE_SELF)     Linux 3.3                \-                 1 ms
getrusage(RUSAGE_SELF)     OpenBSD 5.0              \-                 8 ms
clock()                    FreeBSD 8.2            8 ms                 8 ms
clock()                    Linux 3.2              1 µs                10 ms
times()                    Linux 3.0             10 ms                10 ms
clock()                    OpenBSD 5.0           10 ms                10 ms
times()                    OpenBSD 5.0           10 ms                10 ms
GetProcessTimes()          Windows Seven         16 ms                16 ms
clock()                    SunOS 5.11             1 µs                10 ms
=========================  ================  =========  ===================

Functions
^^^^^^^^^

* Windows: GetProcessTimes(). The precision can be read using
  GetSystemTimeAdjustment().
* clock_gettime(CLOCK_PROCESS_CPUTIME_ID): High-resolution per-process
  timer from the CPU. The precision can be read using clock_getres().
* clock(). The precision is 1 / CLOCKS_PER_SEC.

  * 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.

* getrusage(RUSAGE_SELF) returns a structure of resource usage of the currenet
  process.  ru_utime is use CPU time and ru_stime is the system CPU time.
* times(): structure of process times. The precision is 1 / ticks_per_seconds,
  where ticks_per_seconds is sysconf(_SC_CLK_TCK) or the HZ constant.

Python source code includes a portable library to get the process time (CPU
time): `Tools/pybench/systimes.py
<http://hg.python.org/cpython/file/tip/Tools/pybench/systimes.py>`_.

See also the `QueryProcessCycleTime() function
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms684929(v=vs.85).aspx>`_
(sum of the cycle time of all threads).


Thread time
-----------

The thread time cannot be set.  It is not monotonic: the clocks stop
while the thread is idle.

=========================  ==========  =============  ===============
Name                       Resolution  Include sleep  Include suspend
=========================  ==========  =============  ===============
CLOCK_THREAD_CPUTIME_ID          1 ns  Yes            Epoch changes
GetThreadTimes()               100 ns  No             ?
=========================  ==========  =============  ===============

Examples of clock precision on x86_64:

=========================  ================  ===============  ===================
Name                       Operating system  Precision        Precision in Python
=========================  ================  ===============  ===================
CLOCK_THREAD_CPUTIME_ID    FreeBSD 8.2                  1 µs                 1 µs
CLOCK_THREAD_CPUTIME_ID    Linux 3.2                    1 ns                 6 µs
GetThreadTimes()           Windows Seven               16 ms                16 ms
=========================  ================  ===============  ===================

CLOCK_THREAD_CPUTIME_ID returns a number of CPU cycles, not a number of
seconds.


Functions
^^^^^^^^^

* Windows: GetThreadTimes(). The precision can be read using
  GetSystemTimeAdjustment(). `MSDN documentation of GetThreadTimes()
  <http://msdn.microsoft.com/en-us/library/windows/desktop/ms683237(v=vs.85).aspx>`_.
* clock_gettime(CLOCK_THREAD_CPUTIME_ID): Thread-specific CPU-time
  clock. The precision can be read using of clock_getres().

See also the `QueryThreadCycleTime() function
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms684943(v=vs.85).aspx>`_
(cycle time for the specified thread) and 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.

The elapsed time retrieved by the QueryUnbiasedInterruptTime function
includes only time that the system spends in the working state.
QueryUnbiasedInterruptTime() is not monotonic.

QueryUnbiasedInterruptTime() was introduced in Windows 7.

See also `QueryIdleProcessorCycleTime() function
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms684922(v=vs.85).aspx>`_
(cycle time for the idle thread of each processor)


Sleep
-----

Suspend execution of the process for the given number of seconds.  Sleep is not
affected by system clock update. Sleep is paused during system suspend. For
example, if a process sleeps for 60 seconds and the system is suspended for 30
seconds in the middle of the sleep, the sleep duration is 90 seconds in the
real time.

Sleep can be interrupted by a signal: the function fails with EINTR.

========================  ==========
Name                      Resolution
========================  ==========
nanosleep()               1 ns
clock_nanosleep()         1 ns
usleep()                  1 µs
delay()                   1 µs
sleep()                   1 sec
========================  ==========

Other functions:

========================  ==========
Name                      Resolution
========================  ==========
sigtimedwait()            1 ns
pthread_cond_timedwait()  1 ns
sem_timedwait()           1 ns
select()                  1 µs
epoll()                   1 ms
poll()                    1 ms
WaitForSingleObject()     1 ms
========================  ==========


Functions
^^^^^^^^^

 * sleep(seconds)
 * usleep(microseconds)
 * nanosleep(nanoseconds, remaining):
   `Linux manpage of nanosleep()
   <http://www.kernel.org/doc/man-pages/online/pages/man2/nanosleep.2.html>`_
 * delay(milliseconds)


clock_nanosleep
^^^^^^^^^^^^^^^

clock_nanosleep(clock_id, flags, nanoseconds, remaining): `Linux
manpage of clock_nanosleep()
<http://www.kernel.org/doc/man-pages/online/pages/man2/clock_nanosleep.2.html>`_.

If flags is TIMER_ABSTIME, then request is interpreted as an absolute
time as measured by the clock, clock_id.  If request is less than or
equal to the current value of the clock, then clock_nanosleep()
returns immediately without suspending the calling thread.

POSIX.1 specifies that changing the value of the CLOCK_REALTIME clock
via clock_settime(2) shall have no effect on a thread that is blocked
on a relative clock_nanosleep().


select()
^^^^^^^^

select(nfds, readfds, writefds, exceptfs, timeout).

Since Linux 2.6.28, select() uses high-resolution timers to handle the
timeout.  A process has a "slack" attribute to configure the precision
of the timeout, the default slack is 50 microseconds.  Before Linux
2.6.28, timeouts for select() were handled by the main timing
subsystem at a jiffy-level resolution.  Read also `High- (but not too
high-) resolution timeouts <http://lwn.net/Articles/296578/>`_ and
`Timer slack <http://lwn.net/Articles/369549/>`_.


Other functions
^^^^^^^^^^^^^^^

 * poll(), epoll()
 * sigtimedwait(). POSIX: "If the Monotonic Clock option is supported,
   the CLOCK_MONOTONIC clock shall be used to measure the time
   interval specified by the timeout argument."
 * pthread_cond_timedwait(), pthread_condattr_setclock(). "The default
   value of the clock attribute shall refer to the system clock."
 * sem_timedwait(): "If the Timers option is supported, the timeout
   shall be based on the CLOCK_REALTIME clock.  If the Timers option is
   not supported, the timeout shall be based on the system clock as
   returned by the time() function.  The precision of the timeout
   shall be the precision of the clock on which it is based."
 * WaitForSingleObject(): use the same timer than GetTickCount() with
   the same precision.


System Standby
==============

The ACPI power state "S3" is as system standby mode, also called "Suspend to
RAM". RAM remains powered.

On Windows, the WM_POWERBROADCAST message is send to Windows application
to notify them of power-management events (ex: ower status has changed).

For Mac OS X, read `Registering and unregistering for sleep
and wake notifications
<http://developer.apple.com/library/mac/#qa/qa1340/_index.html>`_ (Technical
Q&A QA1340).


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

Other names for new functions
-----------------------------

time.monotonic():

* time.counter()
* time.seconds()
* time.steady()
* time.timeout_clock()
* time.wallclock(): it is not the system time aka the "wall clock",
  but a monotonic clock with an unspecified starting point

The name "time.try_monotonic()" was also proposed when
time.monotonic() was falling back to the system clock when no
monotonic clock was available.

time.perf_counter():

 * time.hires()
 * time.highres()
 * time.timer()


Only expose operating system clocks
-----------------------------------

To not have to define high-level clocks, which is a difficult task, a
simpler approach is to only expose operating system clocks.
time.clock_gettime() and related clock identifiers were already added
to Python 3.3 for example.


Fallback to system clock
------------------------

If no monotonic clock is available, time.monotonic() falls back to the
system clock.

Issues:

* It is hard to define correctly such function in the documentation:
  is it monotonic? is it steady? is it adjusted?
* Some user want to decide what to do when no monotonic clock is
  available: use another clock, display an error, or do something
  else?


One function choosing the clock from a list of constraints
----------------------------------------------------------

``time.get_clock(*flags)`` with the following flags:

* time.MONOTONIC: clock cannot go backward
* time.STEADY: clock rate is steady and the clock is not adjusted
* time.HIGHRES: clock with the highest precision

time.get_clock() returns None if the clock is found and so calls can
be chained using the or operator.  Example::

    get_time = time.get_clock(time.MONOTONIC) or time.get_clock(time.STEADY) or time.time()
    t = get_time()

Example of flags of system clocks:

* QueryPerformanceCounter: MONOTONIC | HIGHRES
* GetTickCount: MONOTONIC | STEADY
* CLOCK_MONOTONIC: MONOTONIC | STEADY (or only MONOTONIC on Linux)
* CLOCK_MONOTONIC_RAW: MONOTONIC | STEADY
* gettimeofday(): (no flag)


One function with a flag: time.monotonic(fallback=True)
-------------------------------------------------------

* time.monotonic(fallback=True) falls back to the system clock if no
  monotonic clock is available or if the monotonic clock failed.
* time.monotonic(fallback=False) 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 known bugs (only one is not fixed yet)

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

Issues of a hacked monotonic function:

* if the clock is accidentally set forward by an hour and then back
  again, you wouldn't have a useful clock for an hour
* the cache is not shared between processes so different processes
  wouldn't see the same clock value


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 #14309: Deprecate time.clock()
  <http://bugs.python.org/issue14309>`_
* `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>`_
* `Issue #14555: clock_gettime/settime/getres: Add more clock identifiers
  <http://bugs.python.org/issue14555>`_

Libraries exposing monotonic clocks:

* `Java: System.nanoTime
  <http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/System.html#nanoTime()>`_
* `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#g-get-monotonic-time>`_
  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>`_)
* `Monoclock.nano_count()
  <https://github.com/ludios/Monoclock>`_ uses clock_gettime(CLOCK_MONOTONIC)
  and returns a number of nanoseconds
* `monotonic_clock <https://github.com/ThomasHabets/monotonic_clock>`_
* `Perl: Time::HiRes <http://perldoc.perl.org/Time/HiRes.html>`_
  exposes clock_gettime(CLOCK_MONOTONIC)
* `Ruby: AbsoluteTime.now
  <https://github.com/bwbuchanan/absolute_time/>`_: use
  clock_gettime(CLOCK_MONOTONIC), mach_absolute_time() or
  gettimeofday().  "AbsoluteTime.monotonic?" method indicates if
  AbsoluteTime.now is monotonic or not.
* `libpthread
  <http://code.google.com/p/libpthread/>`_: POSIX thread library for Windows
  (`clock.c <http://code.google.com/p/libpthread/source/browse/src/clock.c>`_)
* `Boost.Chrono
  <http://www.boost.org/doc/libs/1_49_0/doc/html/chrono.html>`_ uses:

  * 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

Time:

* `hrtimers - subsystem for high-resolution kernel timers
  <http://www.kernel.org/doc/Documentation/timers/hrtimers.txt>`_
* `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>`_
* `clockspeed <http://cr.yp.to/clockspeed.html>`_ uses a hardware tick
  counter to compensate for a persistently fast or slow system clock
* `Retrieving system time
  <http://en.wikipedia.org/wiki/System_time#Retrieving_system_time>`_
  lists hardware clocks and time functions with their resolution and
  epoch or range
* On Windows, the JavaScript runtime of Firefox interpolates
  GetSystemTimeAsFileTime() with QueryPerformanceCounter() to get an
  higher resolution. See the `Bug 363258 - bad millisecond resolution
  for (new Date).getTime() / Date.now() on Windows
  <https://bugzilla.mozilla.org/show_bug.cgi?id=363258>`_.
* `When microseconds matter
  <http://www.ibm.com/developerworks/library/i-seconds/>`_: How the
  IBM High Resolution Time Stamp Facility accurately measures itty
  bits of time
* `Win32 Performance Measurement Options
  <http://drdobbs.com/windows/184416651>`_ by Matthew Wilson, May 01, 2003
* `Counter Availability and Characteristics for Feed-forward Based Synchronization
  <http://www.cubinlab.ee.unimelb.edu.au/~jrid/Publications/ridoux_ispcs09.pdf>`_
  by Timothy Broomhead, Julien Ridoux, Darryl Veitch (2009)
* System Management Interrupt (SMI) issues:

  * `System Management Interrupt Free Hardware
    <http://linuxplumbersconf.org/2009/slides/Keith-Mannthey-SMI-plumers-2009.pdf>`_
    (Keith Mannthey, 2009)
  * `IBM Real-Time "SMI Free" mode driver
    <http://lwn.net/Articles/318725/>`_
  * `Fixing Realtime problems caused by SMI on Ubuntu
    <http://wiki.linuxcnc.org/cgi-bin/wiki.pl?FixingSMIIssues>`_
  * `[RFC] simple SMI detector
    <http://lwn.net/Articles/316622/>`_
  * `[PATCH 2.6.34-rc3] A nonintrusive SMI sniffer for x86.
    <http://marc.info/?l=linux-kernel&m=127058720921201&w=1>`_


Copyright
=========

This document has been placed in the public domain.



..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8
   End: