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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
     periodically, to run different time-based events.  This page is about the

     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

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

     statclock()    statistics and scheduler events entry.  Called with
                    frequency 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
           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
     tunable/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
     specified 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
     configured 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
     option has no effect - all interrupts are always generating.

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

FreeBSD 11.1-RELEASE-p4         March 13, 2012         FreeBSD 11.1-RELEASE-p4
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