PEP 418: Move Linux clocksource and FreeBSD timecounter
Remove duplicate info on hardware clocks
This commit is contained in:
Victor Stinner
parent
976aae2d30
commit
2d861df8bf
176
pep-0418.txt
176
pep-0418.txt
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@ -368,17 +368,20 @@ Glossary
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Hardware clocks
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===============
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* HPET: An HPET chip consists of a 64-bit up-counter (main counter)
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List of hardware clocks
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-----------------------
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* HPET: An High Precision Event Timer (HPET) chip consists of a 64-bit up-counter (main counter)
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counting at least at 10 MHz and a set of up to 256 comparators (at
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least 3). Each HPET can have up to 32 timers.
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least 3). Each HPET can have up to 32 timers. HPET can cause around 3 seconds of drift per day.
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* TSC (Time Stamp Counter): Historically, the TSC increased with every internal
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processor clock cycle, but now the rate is usually constant (even if the
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processor changes frequency) and usually equals the maximum processor
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frequency. Multiple cores having different TSC values. Hibernation of system
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will reset TSC value. The RDTSC instruction can be used to read this counter.
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CPU frequency scaling for power saving.
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* ACPI PMTMR (Power Management TiMeR): ACPI 24-bit timer with a frequency
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of 3.5 MHz (3,579,545 Hz). HPET can cause around 3 seconds of drift per day.
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* ACPI Power Management Timer: ACPI 24-bit timer with a frequency
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of 3.5 MHz (3,579,545 Hz).
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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
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IBM "Summit" series chipsets (ex: x440). This is available in IA32
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@ -388,6 +391,84 @@ Hardware clocks
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* RTC (Real-time clock). Most RTCs use a crystal oscillator with a frequency of
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32,768 Hz
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Linux clocksource
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-----------------
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There were 4 implementations of the time in the Linux kernel: UTIME
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(1996), timer wheel (1997), HRT (2001) and hrtimers (2007). The
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latter is the result of the "high-res-timers" project started by
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George Anzinger in 2001, with contributions by Thomas Gleixner and
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Douglas Niehaus. hrtimers implementation was merged into Linux
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2.6.21, released in 2007.
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hrtimers supports various clock sources. It sets a priority to each
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source to decide which one will be used. Linux supports the following
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clock sources:
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* tsc
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* hpet
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* pit
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* pmtmr: ACPI Power Management Timer
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* cyclone
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High-resolution timers are not supported on all hardware
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architectures. They are at least provided on x86/x86_64, ARM and
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PowerPC.
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The /sys/devices/system/clocksource/clocksource0 directory contains two useful files:
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* available_clocksource: list of available clock sources
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* current_clocksource: clock source currently used. It is possible to change
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the current clocksource by writing the name of a clocksource into this
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file.
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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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FreeBSD timecounter
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-------------------
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kern.timecounter.choice list available hardward clocks with their priority.
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The sysctl program can be used to change the timecounter. Example::
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# dmesg|grep Timecounter
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Timecounter "i8254" frequency 1193182 Hz quality 0
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Timecounter "ACPI-safe" frequency 3579545 Hz quality 850
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Timecounter "HPET" frequency 100000000 Hz quality 900
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Timecounter "TSC" frequency 3411154800 Hz quality 800
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Timecounters tick every 10.000 msec
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# sysctl kern.timecounter.choice
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kern.timecounter.choice: TSC(800) HPET(900) ACPI-safe(850) i8254(0) dummy(-1000000)
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# sysctl kern.timecounter.hardware="ACPI-fast"
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kern.timecounter.hardware: HPET -> ACPI-fast
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Available clocks:
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* "TSC": Time Stamp Counter of the procesor
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* "HPET": High Precision Event Timer
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* "ACPI-fast": ACPI Power Management timer (fast mode)
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* "ACPI-safe": ACPI Power Management timer (safe mode)
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* "i8254": PIT with Intel 8254 chipset
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The `commit 222222
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<http://svnweb.freebsd.org/base?view=revision&revision=222222>`_ (May 2011)
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decreased ACPI-fast timecounter quality to 900 and increased HPET timecounter
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quality to 950: "HPET on modern platforms usually have better resolution and
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lower latency than ACPI timer".
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Read `Timecounters: Efficient and precise timekeeping in SMP kernels
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<http://phk.freebsd.dk/pubs/timecounter.pdf>`_ by Poul-Henning Kamp (2002) for
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the FreeBSD Project.
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Performances
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------------
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Reading an hardware clock has a cost. The following table compares the
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performance of different hardware clocks on Linux 3.3 with Intel Core i7-2600
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at 3.40GHz (8 cores).
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@ -914,90 +995,6 @@ 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
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(1996), timer wheel (1997), HRT (2001) and hrtimers (2007). The
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latter is the result of the "high-res-timers" project started by
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George Anzinger in 2001, with contributions by Thomas Gleixner and
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Douglas Niehaus. hrtimers implementation was merged into Linux
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2.6.21, released in 2007.
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hrtimers supports various clock sources. It sets a priority to each
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source to decide which one will be used.
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* TSC (Time Stamp Counter): Internal processor clock incremented at
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each processor cycle. Its frequency is the processor frequency and
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so usually higher than 1 GHz. Its priority is 300 by default, but
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falls to 0 if the processor frequency changes and the counter
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becomes unstable.
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* HPET: An High Precision Event Timer (HPET) chip consists of a 64-bit
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up-counter (main counter) counting at least at 10 MHz and a set of up to 256
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comparators (at least 3). Each HPET can have up to 32 timers. HPET can
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cause around 3 seconds of drift per day.
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* PIT (programmable interrupt timer): Intel 8253/8254 chipsets with a
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configurable frequency 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
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of 3.5 MHz (3,579,545 Hz). Its priority is 200 by default, but
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changes to 110 if the chipset is broken and need a software
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workaround.
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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
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IBM "Summit" series chipsets (ex: x440). This is available in IA32
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and IA64 architectures.
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High-resolution timers are not supported on all hardware
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architectures. They are at least provided on x86/x86_64, ARM and
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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.
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It is possible to force a clocksource at runtime by writing its name
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into /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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FreeBSD timers
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--------------
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kern.timecounter.choice list available hardward clocks with their priority.
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The sysctl program can be used to change the timecounter. Example::
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# dmesg|grep Timecounter
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Timecounter "i8254" frequency 1193182 Hz quality 0
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Timecounter "ACPI-safe" frequency 3579545 Hz quality 850
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Timecounter "HPET" frequency 100000000 Hz quality 900
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Timecounter "TSC" frequency 3411154800 Hz quality 800
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Timecounters tick every 10.000 msec
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# sysctl kern.timecounter.choice
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kern.timecounter.choice: TSC(800) HPET(900) ACPI-safe(850) i8254(0) dummy(-1000000)
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# sysctl kern.timecounter.hardware="ACPI-fast"
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kern.timecounter.hardware: HPET -> ACPI-fast
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Available clocks:
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* "TSC": Time Stamp Counter of the procesor
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* "HPET": High Precision Event Timer
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* "ACPI-fast": ACPI Power Management timer (fast mode)
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* "ACPI-safe": ACPI Power Management timer (safe mode)
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* "i8254": PIT with Intel 8254 chipset
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The `commit 222222
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<http://svnweb.freebsd.org/base?view=revision&revision=222222>`_ (May 2011)
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decreased ACPI-fast timecounter quality to 900 and increased HPET timecounter
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quality to 950: "HPET on modern platforms usually have better resolution and
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lower latency than ACPI timer".
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Read `Timecounters: Efficient and precise timekeeping in SMP kernels
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<http://phk.freebsd.dk/pubs/timecounter.pdf>`_ by Poul-Henning Kamp (2002) for
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the FreeBSD Project.
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Sleep, suspend and monotonic time
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=================================
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@ -1062,6 +1059,7 @@ poll() 1 ms
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WaitForSingleObject() 1 ms
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======================== ==========
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Classic functions
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-----------------
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@ -1072,6 +1070,7 @@ Classic functions
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<http://www.kernel.org/doc/man-pages/online/pages/man2/nanosleep.2.html>`_
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* delay(milliseconds)
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clock_nanosleep
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---------------
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@ -1088,6 +1087,7 @@ POSIX.1 specifies that changing the value of the CLOCK_REALTIME clock via
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clock_settime(2) shall have no effect on a thread that is blocked on a relative
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clock_nanosleep().
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select()
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--------
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