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.Dd March 13, 2012 26.Dt EVENTTIMERS 4 27.Os 28.Sh NAME 29.Nm eventtimers 30.Nd kernel event timers subsystem 31.Sh SYNOPSIS 32Kernel uses several types of time-related devices, such as: real time clocks, 33time counters and event timers. 34Real time clocks responsible for tracking real world time, mostly when system 35is down. 36Time counters are responsible for generation of monotonically increasing 37timestamps for precise uptime tracking purposes, when system is running. 38Event timers are responsible for generating interrupts at specified time or 39periodically, to run different time-based events. 40This page is about the last. 41.Sh DESCRIPTION 42Kernel uses time-based events for many different purposes: scheduling, 43statistics, time keeping, profiling and many other things, based on 44.Xr callout 9 45mechanism. 46These purposes now grouped into three main callbacks: 47.Bl -tag -width ".Fn hardclock" 48.It Fn hardclock 49.Xr callout 9 50and timekeeping events entry. 51Called with frequency defined by 52.Va hz 53variable, 54usually 1000Hz. 55.It Fn statclock 56statistics and scheduler events entry. 57Called with frequency about 128Hz. 58.It Fn profclock 59profiler events entry. 60When enabled, called with frequency about 8KHz. 61.El 62.Pp 63Different platforms provide different kinds of timer hardware. 64The goal of the event timers subsystem is to provide unified way to control 65that hardware, and to use it, supplying kernel with all required time-based 66events. 67.Pp 68Each driver implementing event timers, registers them at the subsystem. 69It is possible to see the list of present event timers, like this, via 70.Va kern.eventtimer 71sysctl: 72.Bd -literal 73kern.eventtimer.choice: HPET(550) LAPIC(400) i8254(100) RTC(0) 74kern.eventtimer.et.LAPIC.flags: 15 75kern.eventtimer.et.LAPIC.frequency: 0 76kern.eventtimer.et.LAPIC.quality: 400 77kern.eventtimer.et.i8254.flags: 1 78kern.eventtimer.et.i8254.frequency: 1193182 79kern.eventtimer.et.i8254.quality: 100 80kern.eventtimer.et.RTC.flags: 17 81kern.eventtimer.et.RTC.frequency: 32768 82kern.eventtimer.et.RTC.quality: 0 83kern.eventtimer.et.HPET.flags: 7 84kern.eventtimer.et.HPET.frequency: 14318180 85kern.eventtimer.et.HPET.quality: 550 86.Ed 87.Pp 88where: 89.Bl -inset 90.It Va kern.eventtimer.et. Ns Ar X Ns Va .flags 91is a 92bitmask, defining event timer capabilities: 93.Bl -tag -offset indent -width indent -compact 94.It 1 95periodic mode supported, 96.It 2 97one-shot mode supported, 98.It 4 99timer is per-CPU, 100.It 8 101timer may stop when CPU goes to sleep state, 102.It 16 103timer supports only power-of-2 divisors. 104.El 105.It Va kern.eventtimer.et. Ns Ar X Ns Va .frequency 106is a 107timer base frequency, 108.It Va kern.eventtimer.et. Ns Ar X Ns Va .quality 109is an 110integral value, defining how good is this timer, comparing to others. 111.El 112.Pp 113Timers management code of the kernel chooses one timer from that list. 114Current choice can be read and affected via 115.Va kern.eventtimer.timer 116tunable/sysctl. 117Several other tunables/sysctls are affecting how exactly this timer is used: 118.Bl -inset 119.It Va kern.eventtimer.periodic 120allows to choose periodic and one-shot operation mode. 121In periodic mode, periodic interrupts from timer hardware are taken as the 122only source of time for time events. 123One-shot mode instead uses currently selected time counter to precisely 124schedule all needed events and programs event timer to generate interrupt 125exactly in specified time. 126Default value depends of chosen timer capabilities, but one-shot mode is 127preferred, until other is forced by user or hardware. 128.It Va kern.eventtimer.singlemul 129in periodic mode specifies how much times higher timer frequency should be, 130to not strictly alias 131.Fn hardclock 132and 133.Fn statclock 134events. 135Default values are 1361, 2 or 4, depending on configured HZ value. 137.It Va kern.eventtimer.idletick 138makes each CPU to receive every timer interrupt independently of whether they 139busy or not. 140By default this options is disabled. 141If chosen timer is per-CPU 142and runs in periodic mode, this option has no effect - all interrupts are 143always generating. 144.El 145.Sh SEE ALSO 146.Xr apic 4 , 147.Xr atrtc 4 , 148.Xr attimer 4 , 149.Xr hpet 4 , 150.Xr timecounters 4 , 151.Xr eventtimers 9 152