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EVENTTIMERS(4)	       FreeBSD Kernel Interfaces Manual		EVENTTIMERS(4)

     eventtimers -- kernel event timers	subsystem

     Kernel uses several types of time-related devices,	such as: real time
     clocks, time counters and event timers.  Real time	clocks responsible for
     tracking real world time, mostly when system is down.  Time counters are
     responsible for generation	of monotonically increasing timestamps for
     precise uptime tracking purposes, when system is running.	Event timers
     are responsible for generating interrupts at specified time or periodi-
     cally, to run different time-based	events.	 This page is about the	last.

     Kernel uses time-based events for many different purposes:	scheduling,
     statistics, time keeping, profiling and many other	things,	based on
     callout(9)	mechanism.  These purposes now grouped into three main call-

     hardclock()   callout(9) and timekeeping events entry. Called with	fre-
		   quency defined by hz	variable, usually 1000Hz.

     statclock()   statistics and scheduler events entry. Called with fre-
		   quency about	128Hz.

     profclock()   profiler events entry. When enabled,	called with frequency
		   about 8KHz.
     Different platforms provide different kinds of timer hardware.  The goal
     of	the event timers subsystem is to provide unified way to	control	that
     hardware, and to use it, supplying	kernel with all	required time-based

     Each driver implementing event timers, registers them at the subsystem.
     It	is possible to see the list of present event timers, like this,	via
     kern.eventtimer sysctl:

     kern.eventtimer.choice: HPET(550) LAPIC(400) i8254(100) RTC(0) 15 0 400 1 1193182 100 17 32768 0 7	14318180 550
     , where:
		   bitmask, defining event timer capabilities:
		   1		 periodic mode supported,
		   2		 one-shot mode supported,
		   4		 timer is per-CPU,
		   8		 timer may stop	when CPU goes to sleep state,
		   16		 timer supports	only power-of-2	divisors.
		   timer base frequency,
		   integral value, defining how	good is	this timer, comparing
		   to others.

     Timers management code of the kernel chooses one timer from that list.
     Current choice can	be read	and affected via kern.eventtimer.timer tun-
     able/sysctl.  Several other tunables/sysctls are affecting	how exactly
     this timer	is used:

		   allows to choose periodic and one-shot operation mode.  In
		   periodic mode, periodic interrupts from timer hardware are
		   taken as the	only source of time for	time events.  One-shot
		   mode	instead	uses currently selected	time counter to	pre-
		   cisely schedule all needed events and programs event	timer
		   to generate interrupt exactly in specified time.  Default
		   value depends of chosen timer capabilities, but one-shot
		   mode	is preferred, until other is forced by user or hard-

		   in periodic mode specifies how much times higher timer fre-
		   quency should be, to	not strictly alias hardclock() and
		   statclock() events. Default values are 1, 2 or 4, depending
		   on configured HZ value.

		   makes each CPU to receive every timer interrupt indepen-
		   dently of whether they busy or not. By default this options
		   is disabled.	If chosen timer	is per-CPU and runs in peri-
		   odic	mode, this option has no effect	- all interrupts are
		   always generating.

     attimer(4), atrtc(4), hpet(4)

FreeBSD	10.2		      September	15, 2010		  FreeBSD 10.2


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