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)

Debian                                          December 20, 2011                                TIMECOUNTERS(4)