NAME

     timecounters — kernel time counters 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 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 second.

DESCRIPTION

     Time counters are the lowest level of time tracking in kernel.  They provide monotonically
     increasing timestamps with known width and update frequency.  They can overflow, drift, etc
     and so in raw form used only in very limited performance-critical places like process
     scheduler.

     More usable time is created by scaling the values read from the selected time counter and
     combining it with some offset, regularly updated by tc_windup() on hardclock() invocation.

     Different platforms provide different kinds of timer hardware.  The goal of the time
     counters subsystem is to provide unified way to access that hardware.

     Each driver implementing time counters, registers them at the subsystem.  It is possible to
     see the list of present time counters, like this, via kern.timecounter sysctl:

     kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
     kern.timecounter.tc.ACPI-fast.mask: 16777215
     kern.timecounter.tc.ACPI-fast.counter: 13467909
     kern.timecounter.tc.ACPI-fast.frequency: 3579545
     kern.timecounter.tc.ACPI-fast.quality: 900
     kern.timecounter.tc.i8254.mask: 65535
     kern.timecounter.tc.i8254.counter: 62692
     kern.timecounter.tc.i8254.frequency: 1193182
     kern.timecounter.tc.i8254.quality: 0
     kern.timecounter.tc.HPET.mask: 4294967295
     kern.timecounter.tc.HPET.counter: 3013495652
     kern.timecounter.tc.HPET.frequency: 14318180
     kern.timecounter.tc.HPET.quality: 950
     kern.timecounter.tc.TSC-low.mask: 4294967295
     kern.timecounter.tc.TSC-low.counter: 4067509463
     kern.timecounter.tc.TSC-low.frequency: 11458556
     kern.timecounter.tc.TSC-low.quality: -100

     where:

     kern.timecounter.tc.X.mask is a bitmask, defining valid counter bits,

     kern.timecounter.tc.X.counter is a present counter value,

     kern.timecounter.tc.X.frequency is a counter update frequency,

     kern.timecounter.tc.X.quality is an integral value, defining how good is this time counter,
     comparing to others.  Negative value means that this time counter is broken and should not
     be used.

     Time management code of the kernel chooses one time counter from that list.  Current choice
     can be read and affected via kern.timecounter.hardware tunable/sysctl.

SEE ALSO

     attimer(4), eventtimers(4), hpet(4)