1.\" Copyright (c) 2010 Alexander Motin <mav@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd September 15, 2010 28.Dt EVENTTIMERS 4 29.Os 30.Sh NAME 31.Nm eventtimers 32.Nd kernel event timers subsystem 33.Sh SYNOPSIS 34Kernel uses several types of time-related devices, such as: real time clocks, 35time counters and event timers. 36Real time clocks responsible for tracking real world time, mostly when system 37is down. 38Time counters are responsible for generation of monotonically increasing 39timestamps for precise uptime tracking purposes, when system is running. 40Event timers are responsible for generating interrupts at specified time or 41periodically, to run different time-based events. 42This page is about the last. 43.Sh DESCRIPTION 44Kernel uses time-based events for many different purposes: scheduling, 45statistics, time keeping, profiling and many other things, based on 46.Xr callout 9 47mechanism. 48These purposes now grouped into three main callbacks: 49.Bl -tag 50.It hardclock() 51.Xr callout 9 52and timekeeping events entry. Called with frequency defined by hz variable, 53usually 1000Hz. 54.It statclock() 55statistics and scheduler events entry. Called with frequency about 128Hz. 56.It profclock() 57profiler events entry. When enabled, called with frequency about 8KHz. 58.El 59Different platforms provide different kinds of timer hardware. 60The goal of the event timers subsystem is to provide unified way to control 61that hardware, and to use it, supplying kernel with all required time-based 62events. 63.Pp 64Each driver implementing event timers, registers them at the subsystem. 65It is possible to see the list of present event timers, like this, via 66.Va kern.eventtimer 67sysctl: 68.Bd -literal 69kern.eventtimer.choice: HPET(550) LAPIC(400) i8254(100) RTC(0) 70kern.eventtimer.et.LAPIC.flags: 15 71kern.eventtimer.et.LAPIC.frequency: 0 72kern.eventtimer.et.LAPIC.quality: 400 73kern.eventtimer.et.i8254.flags: 1 74kern.eventtimer.et.i8254.frequency: 1193182 75kern.eventtimer.et.i8254.quality: 100 76kern.eventtimer.et.RTC.flags: 17 77kern.eventtimer.et.RTC.frequency: 32768 78kern.eventtimer.et.RTC.quality: 0 79kern.eventtimer.et.HPET.flags: 7 80kern.eventtimer.et.HPET.frequency: 14318180 81kern.eventtimer.et.HPET.quality: 550 82.Ed 83, where: 84.Bl -tag 85.It Va kern.eventtimer.et. Ns Ar X Ns Va .flags 86bitmask, defining event timer capabilities: 87.Bl -tag -compact 88.It 1 89periodic mode supported, 90.It 2 91one-shot mode supported, 92.It 4 93timer is per-CPU, 94.It 8 95timer may stop when CPU goes to sleep state, 96.It 16 97timer supports only power-of-2 divisors. 98.El 99.It Va kern.eventtimer.et. Ns Ar X Ns Va .frequency 100timer base frequency, 101.It Va kern.eventtimer.et. Ns Ar X Ns Va .quality 102integral value, defining how good is this timer, comparing to others. 103.El 104.Pp 105Timers management code of the kernel chooses one timer from that list. 106Current choice can be read and affected via 107.Va kern.eventtimer.timer 108tunable/sysctl. 109Several other tunables/sysctls are affecting how exactly this timer is used: 110.Bl -tag 111.It Va kern.eventtimer.periodic 112allows to choose periodic and one-shot operation mode. 113In periodic mode, periodic interrupts from timer hardware are taken as the 114only source of time for time events. 115One-shot mode instead uses currently selected time counter to precisely 116schedule all needed events and programs event timer to generate interrupt 117exactly in specified time. 118Default value depends of chosen timer capabilities, but one-shot mode is 119preferred, until other is forced by user or hardware. 120.It Va kern.eventtimer.singlemul 121in periodic mode specifies how much times higher timer frequency should be, 122to not strictly alias hardclock() and statclock() events. Default values are 1231, 2 or 4, depending on configured HZ value. 124.It Va kern.eventtimer.idletick 125makes each CPU to receive every timer interrupt independently of whether they 126busy or not. By default this options is disabled. If chosen timer is per-CPU 127and runs in periodic mode, this option has no effect - all interrupts are 128always generating. 129.El 130.Sh SEE ALSO 131.Xr attimer 4 , 132.Xr atrtc 4 , 133.Xr hpet 4 134