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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:
                   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
     tunable/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
                   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.

                   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.

                   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.

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

FreeBSD 11.0-PRERELEASE       September 15, 2010       FreeBSD 11.0-PRERELEASE


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