NAME

     eventtimers — kernel event timers subsystem

SYNOPSIS

     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.

DESCRIPTION

     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)
     kern.eventtimer.et.LAPIC.flags: 15
     kern.eventtimer.et.LAPIC.frequency: 0
     kern.eventtimer.et.LAPIC.quality: 400
     kern.eventtimer.et.i8254.flags: 1
     kern.eventtimer.et.i8254.frequency: 1193182
     kern.eventtimer.et.i8254.quality: 100
     kern.eventtimer.et.RTC.flags: 17
     kern.eventtimer.et.RTC.frequency: 32768
     kern.eventtimer.et.RTC.quality: 0
     kern.eventtimer.et.HPET.flags: 7
     kern.eventtimer.et.HPET.frequency: 14318180
     kern.eventtimer.et.HPET.quality: 550

     where:

     kern.eventtimer.et.X.flags 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.

     kern.eventtimer.et.X.frequency is a timer base frequency,

     kern.eventtimer.et.X.quality 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.

SEE ALSO

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