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)

Debian                                           March 13, 2012                                   EVENTTIMERS(4)