PEP 418: more data about OS monotonic clocks
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Victor Stinner
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pep-0418.txt
79
pep-0418.txt
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@ -180,6 +180,25 @@ Clocks
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Monotonic
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---------
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Summary
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^^^^^^^
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Table summarizing all monotonic clocks:
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========================= =============== ================ =================
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Name Resolution Adjusted by NTP? Action on suspend
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========================= =============== ================ =================
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CLOCK_MONOTONIC_RAW 1 ns No ?
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gethrtime 1 ns ? ?
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mach_absolute_time() 1 ns ? ?
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CLOCK_HIGHRES 1 ns ? ?
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CLOCK_MONOTONIC 1 ns Yes on Linux Stopped on Linux
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QueryPerformanceCounter() 0.3 ns - 5 ns No ?
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GetTickCount[64]() 1 ms - 15 ms No ?
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timeGetTime() 1 ms - 15 ms ? ?
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========================= =============== ================ =================
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mach_absolute_time
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^^^^^^^^^^^^^^^^^^
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@ -246,6 +265,10 @@ Windows: QueryPerformanceCounter
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High-resolution performance counter. It is monotonic.
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QueryPerformanceFrequency() gives its frequency.
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It has much higher resolution, but has lower long term accuracy than
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GetTickCount() and timeGetTime() clocks. For example, it will drift compared to
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the low precision clocks.
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Documentation:
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* `MSDN: QueryPerformanceCounter() documentation
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@ -258,13 +281,18 @@ Hardware clocks used by QueryPerformanceCounter:
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* Windows XP: RDTSC instruction of Intel processors, the clock frequency is
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the frequency of the processor (between 200 MHz and 3 GHz, usually greater
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than 1 GHz nowadays)
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* Windows 2000: ACPI power management timer, frequency = 3,549,545 Hz
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* Windows 2000: ACPI power management timer, frequency = 3,549,545 Hz. It can
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be forced through the "/usepmtimer" flag in boot.ini
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* Windows 95/98: 8245 PIT chipset, frequency = 1,193,181 Hz
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QueryPerformanceFrequency() should only be called once: the frequency will not
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change while the system is running. It fails if the installed hardware does not
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support a high-resolution performance counter.
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QueryPerformanceCounter() cannot be adjusted: `SetSystemTimeAdjustment()
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<http://msdn.microsoft.com/en-us/library/windows/desktop/ms724943(v=vs.85).aspx>`_
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does only adjust the system time.
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Bugs:
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* The performance counter value may unexpectedly leap forward because of a
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@ -277,6 +305,7 @@ Bugs:
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#8707 <https://www.virtualbox.org/ticket/8707>`_).
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* Windows XP had a bug (see `KB896256`_): on a multiprocessor computer,
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QueryPerformanceCounter() returned a different value for each processor.
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The bug was fixed in Windows XP SP2.
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.. _KB896256: http://support.microsoft.com/?id=896256
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.. _KB274323: http://support.microsoft.com/?id=274323
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@ -285,7 +314,10 @@ Bugs:
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Windows: GetTickCount(), GetTickCount64()
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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GetTickCount() and GetTickCount64() are monotonic and cannot fail.
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GetTickCount() and GetTickCount64() are monotonic, cannot fail and are not
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adjusted by SetSystemTimeAdjustment(). MSDN documentation:
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`GetTickCount() <http://msdn.microsoft.com/en-us/library/windows/desktop/ms724408(v=vs.85).aspx>`_,
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`GetTickCount64() <http://msdn.microsoft.com/en-us/library/windows/desktop/ms724411(v=vs.85).aspx>`_.
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GetTickCount64() was added to Windows Vista and Windows Server 2008.
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@ -297,6 +329,29 @@ There are applications using this undocumented function, example:
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`Timer Resolution <http://www.lucashale.com/timer-resolution/>`_.
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Windows: timeGetTime
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^^^^^^^^^^^^^^^^^^^^
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The timeGetTime function retrieves the system time, in milliseconds. The system
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time is the time elapsed since Windows was started. Read the `timeGetTime()
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documentation
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<http://msdn.microsoft.com/en-us/library/windows/desktop/dd757629(v=vs.85).aspx>`_.
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The return type of timeGetTime() is a 32-bit unsigned integer. As
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GetTickCount(), timeGetTime() rolls over after 2^32 milliseconds (49.7 days).
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The default precision of the timeGetTime function can be five milliseconds or
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more, depending on the machine.
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timeBeginPeriod() can be used to increase the precision of timeGetTime() up to
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1 millisecond.
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.. note::
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timeGetTime() and timeBeginPeriod() are part the Windows multimedia library
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and so require to link the program with winmm or to load dynamically the
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library.
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Solaris: CLOCK_HIGHRES
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^^^^^^^^^^^^^^^^^^^^^^
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@ -329,7 +384,7 @@ The system time can be read using GetSystemTimeAsFileTime(), ftime() and
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time().
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The system time resolution can be read using GetSystemTimeAdjustment(). The
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accurary is usually between 0.5 millisecond and 15 milliseconds. Resolution:
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accurary is usually between 1 millisecond and 15 milliseconds. Resolution:
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* GetSystemTimeAsFileTime(): 100 nanoseconds
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* ftime(): 1 millisecond
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@ -495,7 +550,7 @@ Librairies exposing monotonic clocks:
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* `Qt library: QElapsedTimer
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<http://qt-project.org/doc/qt-4.8/qelapsedtimer.html>`_
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* `glib library: g_get_monotonic_time ()
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<http://developer.gnome.org/glib/2.30/glib-Date-and-Time-Functions.html>`_
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<http://developer.gnome.org/glib/2.30/glib-Date-and-Time-Functions.html#g-get-monotonic-time>`_
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uses GetTickCount64()/GetTickCount() on Windows,
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clock_gettime(CLOCK_MONOTONIC) on UNIX or falls back to the system clock
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* `python-monotonic-time
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@ -512,7 +567,7 @@ Librairies exposing monotonic clocks:
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"AbsoluteTime.monotonic?" method indicates if AbsoluteTime.now is monotonic
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or not.
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Related documents:
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Time:
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* `C++ Timeout Specification
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<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3128.html>`_
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@ -520,4 +575,18 @@ Related documents:
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<http://msdn.microsoft.com/en-us/library/ee417693.aspx>`_
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* `Implement a Continuously Updating, High-Resolution Time Provider for Windows
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<http://msdn.microsoft.com/en-us/magazine/cc163996.aspx>`_
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* `clockspeed <http://cr.yp.to/clockspeed.html>`_ uses a hardware tick counter
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to compensate for a persistently fast or slow system clock
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* `Retrieving system time
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<http://en.wikipedia.org/wiki/System_time#Retrieving_system_time>`_
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lists hardware clocks and time functions with their resolution
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and epoch or range
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* On Windows, the JavaScript runtime of Firefox interpolates
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GetSystemTimeAsFileTime() with QueryPerformanceCounter() to get
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an higher resolution. See the `Bug 363258 - bad millisecond resolution for
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(new Date).getTime() / Date.now() on Windows
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<https://bugzilla.mozilla.org/show_bug.cgi?id=363258>`_.
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* `When microseconds matter
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<http://www.ibm.com/developerworks/library/i-seconds/>`_: How the IBM High
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Resolution Time Stamp Facility accurately measures itty bits of time
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