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     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 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 callbacks:

     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 events.

     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 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. 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)