[Python-checkins] peps: PEP 418: Move Linux clocksource and FreeBSD timecounter
victor.stinner
python-checkins at python.org
Thu Apr 12 01:22:56 CEST 2012
http://hg.python.org/peps/rev/81e4519bdde9
changeset: 4228:81e4519bdde9
user: Victor Stinner <victor.stinner at gmail.com>
date: Thu Apr 12 01:22:23 2012 +0200
summary:
PEP 418: Move Linux clocksource and FreeBSD timecounter
Remove duplicate info on hardware clocks
files:
pep-0418.txt | 174 +++++++++++++++++++-------------------
1 files changed, 88 insertions(+), 86 deletions(-)
diff --git a/pep-0418.txt b/pep-0418.txt
--- a/pep-0418.txt
+++ b/pep-0418.txt
@@ -368,17 +368,20 @@
Hardware clocks
===============
-* HPET: An HPET chip consists of a 64-bit up-counter (main counter)
+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.
+ 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 PMTMR (Power Management TiMeR): ACPI 24-bit timer with a frequency
- of 3.5 MHz (3,579,545 Hz). HPET can cause around 3 seconds of drift per day.
+* 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
@@ -388,6 +391,84 @@
* 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 hardward 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 procesor
+ * "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.
+
+
+Performances
+------------
+
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).
@@ -914,90 +995,6 @@
QueryUnbiasedInterruptTime() was introduced in Windows 7.
-Linux timers
-------------
-
-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.
-
-* TSC (Time Stamp Counter): Internal processor clock incremented at
- each processor cycle. Its frequency is the processor frequency and
- so usually higher than 1 GHz. Its priority is 300 by default, but
- falls to 0 if the processor frequency changes and the counter
- becomes unstable.
-* 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.
-* PIT (programmable interrupt timer): Intel 8253/8254 chipsets with a
- configurable frequency in range 18.2 Hz - 1.2 MHz. Linux uses the
- frequency 1,193,181.8 Hz. It is a 16-bit counter.
-* PMTMR (power management timer): ACPI 24-bit timer with a frequency
- of 3.5 MHz (3,579,545 Hz). Its priority is 200 by default, but
- changes to 110 if the chipset is broken and need a software
- workaround.
-* 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.
-
-High-resolution timers are not supported on all hardware
-architectures. They are at least provided on x86/x86_64, ARM and
-PowerPC.
-
-The list of available clock sources can be read in
-/sys/devices/system/clocksource/clocksource0/available_clocksource.
-It is possible to force a clocksource at runtime by writing its name
-into /sys/devices/system/clocksource/clocksource0/current_clocksource.
-/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 timers
---------------
-
-kern.timecounter.choice list available hardward 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 procesor
- * "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.
-
-
Sleep, suspend and monotonic time
=================================
@@ -1062,6 +1059,7 @@
WaitForSingleObject() 1 ms
======================== ==========
+
Classic functions
-----------------
@@ -1072,6 +1070,7 @@
<http://www.kernel.org/doc/man-pages/online/pages/man2/nanosleep.2.html>`_
* delay(milliseconds)
+
clock_nanosleep
---------------
@@ -1088,6 +1087,7 @@
clock_settime(2) shall have no effect on a thread that is blocked on a relative
clock_nanosleep().
+
select()
--------
--
Repository URL: http://hg.python.org/peps
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