108 lines
4.3 KiB
Groff
108 lines
4.3 KiB
Groff
.\" Copyright (c) 2011 Alexander Motin <mav@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd April 12, 2014
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.Dt TIMECOUNTERS 4
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.Os
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.Sh NAME
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.Nm timecounters
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.Nd kernel time counters subsystem
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.Sh SYNOPSIS
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The kernel uses several types of time-related devices, such as: real time clocks,
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time counters and event timers.
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Real time clocks are responsible for tracking real world time, mostly when the system
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is down.
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Time counters are responsible for tracking purposes, when the system is running.
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Event timers are responsible for generating interrupts at a specified time or
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periodically, to run different time-based events.
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This page is about the second.
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.Sh DESCRIPTION
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Time counters are the lowest level of time tracking in the kernel.
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They provide monotonically increasing timestamps with known width and
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update frequency.
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They can overflow, drift, etc and so in raw form can be used only in very limited
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performance-critical places like the process scheduler.
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.Pp
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More usable time is created by scaling the values read from the selected
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time counter and combining it with some offset, regularly updated by
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.Fn tc_windup
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on
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.Fn hardclock
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invocation.
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.Pp
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Different platforms provide different kinds of timer hardware.
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The goal of the time counters subsystem is to provide a unified way to access
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that hardware.
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.Pp
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Each driver implementing time counters registers them with the subsystem.
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It is possible to see the list of present time counters, via the
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.Va kern.timecounter
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.Xr sysctl 8
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variable:
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.Bd -literal
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kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
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kern.timecounter.tc.ACPI-fast.mask: 16777215
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kern.timecounter.tc.ACPI-fast.counter: 13467909
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kern.timecounter.tc.ACPI-fast.frequency: 3579545
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kern.timecounter.tc.ACPI-fast.quality: 900
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kern.timecounter.tc.i8254.mask: 65535
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kern.timecounter.tc.i8254.counter: 62692
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kern.timecounter.tc.i8254.frequency: 1193182
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kern.timecounter.tc.i8254.quality: 0
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kern.timecounter.tc.HPET.mask: 4294967295
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kern.timecounter.tc.HPET.counter: 3013495652
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kern.timecounter.tc.HPET.frequency: 14318180
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kern.timecounter.tc.HPET.quality: 950
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kern.timecounter.tc.TSC-low.mask: 4294967295
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kern.timecounter.tc.TSC-low.counter: 4067509463
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kern.timecounter.tc.TSC-low.frequency: 11458556
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kern.timecounter.tc.TSC-low.quality: -100
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.Ed
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.Pp
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The output nodes are defined as follows:
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.Bl -inset
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .mask
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is a bitmask, defining valid counter bits,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .counter
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is a present counter value,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .frequency
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is a counter update frequency,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .quality
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is an integral value, defining the quality of this time counter
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compared to others.
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A negative value means this time counter is broken and should not be used.
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.El
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.Pp
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The time management code of the kernel chooses one time counter from that list.
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The current choice can be read and affected via the
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.Va kern.timecounter.hardware
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tunable/sysctl.
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.Sh SEE ALSO
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.Xr attimer 4 ,
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.Xr eventtimers 4 ,
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.Xr ffclock 4 ,
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.Xr hpet 4
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