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EVENTTIMERS(4)		 BSD 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:	is a bitmask, defining event timer capabili-
	   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. is a timer base frequency, is an	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:

     kern.eventtimer.periodic 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 precisely schedule	all needed
     events and	programs event timer to	generate interrupt exactly in speci-
     fied time.	 Default value depends of chosen timer capabilities, but one-
     shot mode is preferred, until other is forced by user or hardware.

     kern.eventtimer.singlemul in periodic mode	specifies how much times
     higher timer frequency should be, to not strictly alias hardclock() and
     statclock() events.  Default values are 1,	2 or 4,	depending on config-
     ured HZ value.

     kern.eventtimer.idletick makes each CPU to	receive	every timer interrupt
     independently of whether they busy	or not.	 By default this options is
     disabled.	If chosen timer	is per-CPU and runs in periodic	mode, this op-
     tion has no effect	- all interrupts are always generating.

     apic(4), atrtc(4),	attimer(4), hpet(4), timecounters(4), eventtimers(9)

BSD				March 13, 2012				   BSD


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