PEP 418: split resolution/accuracy in the able; add a section on Linux timers
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Victor Stinner
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pep-0418.txt
83
pep-0418.txt
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@ -186,18 +186,29 @@ Summary
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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 Stopped
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gethrtime 1 ns No Not stopped
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mach_absolute_time() 1 ns No ?
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CLOCK_HIGHRES 1 ns No ?
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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 Accuracy issue
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GetTickCount[64]() 1 ms - 15 ms No Include suspend time
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timeGetTime() 1 ms - 15 ms No ?
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========================= =============== ================ ====================
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========================= =============== =============== ================ ====================
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Name Resolution Accuracy Adjusted by NTP? Action on suspend
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========================= =============== =============== ================ ====================
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CLOCK_MONOTONIC_RAW 1 ns ? No Stopped
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gethrtime 1 ns ? No Not stopped
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mach_absolute_time() 1 ns ? No ?
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CLOCK_HIGHRES 1 ns ? No ?
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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 Accuracy issue
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GetTickCount[64]() 1 ms 1 ms - 15 ms No Include suspend time
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timeGetTime() 1 ms 1 ms - 15 ms No ?
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========================= =============== =============== ================ ====================
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The resolution is the smallest difference between two timestamps supported by
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the format used by the clock. For example, clock_gettime() uses a timespec
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structure which has two integer fileds, tv_sec and tv_nsec, so the resolution
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is 1 nanosecond.
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The accuracy is the effective smallest difference of two timestamps of the
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clock. It does not reflect the stability the clock rate. For example,
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QueryPerformanceCounter() has a good accuracy but is known to not have a steady
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rate.
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mach_absolute_time
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^^^^^^^^^^^^^^^^^^
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@ -351,7 +362,7 @@ 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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1 millisecond, but it negatively affects power consumption.
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.. note::
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timeGetTime() and timeBeginPeriod() are part the Windows multimedia library
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@ -477,6 +488,50 @@ QueryUnbiasedInterruptTime() is not monotonic.
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QueryUnbiasedInterruptTime() was introduced in Windows 7.
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Linux timers
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------------
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There were 4 implementations of the time in the Linux kernel: UTIME (1996),
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timer wheel (1997), HRT (2001) and hrtimers (2007). The later is the result of
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the "high-res-timers" project started by George Anzinger in 2001, contributed
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by Thomas Gleixner and Douglas Niehaus. hrtimers implementation was merged into
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Linux 2.6.21 released in 2007.
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hrtimers supports various clock sources. It sets a priority to each source to
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decide which one will be used.
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* TSC (Time Stamp Counter): Internal processor clock incremented at each
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processor cycle. Its frequency is the processor frequency and so usually
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higher than 1 GHz. Its priority is 300 by default, but falls to 0 if the
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processor frequency changes and the counter becomes unstable.
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* HPET: An HPET chip consists of a 64-bit up-counter (main counter) counting
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at least at 10 MHz and a set of up to 256 comparators (at least 3). Each
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HPET can have up to 32 timers.
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* PIT (programmable interrupt timer): Intel 8253/8254 chipsets with a
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configurable frequecency in range 18.2 Hz - 1.2 MHz. Linux uses the
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frequency: 1,193,181.8 Hz. It is a 16-bit counter.
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* PMTMR (power management timer): ACPI 24-bit timer with a frequency of 3.5
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MHz (3,579,545 Hz). Its priority is 200 by default, but changes to 110 if
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the chipset is broken and need a software workaround. HPET can cause around
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3 seconds of drift per day.
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* Cyclone: The Cyclone timer uses a 32-bit counter on IBM Extended
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X-Architecture (EXA) chipsets which include computers that use the IBM
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"Summit" series chipsets (ex: x440). This is available in IA32 and IA64
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architectures.
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High-resolution timers are not supported on all hardware architectures. They
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are at least provided on x86/x86_64, ARM and PowerPC.
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The list of available clock sources can be read in
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/sys/devices/system/clocksource/clocksource0/available_clocksource. It is
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possible to force a clocksource at runtime by writing its name into
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/sys/devices/system/clocksource/clocksource0/current_clocksource.
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/proc/timer_list contains the list of all hardware timers.
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Read also the `time(7) manual page
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<http://www.kernel.org/doc/man-pages/online/pages/man7/time.7.html>`_:
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"overview of time and timers".
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Alternatives: API design
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========================
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@ -584,6 +639,8 @@ Librairies exposing monotonic clocks:
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Time:
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* `hrtimers - subsystem for high-resolution kernel timers
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<http://www.kernel.org/doc/Documentation/timers/hrtimers.txt>`_
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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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* `Windows: Game Timing and Multicore Processors
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