NAME

       timecounters — kernel time counters subsystem

SYNOPSIS

       The kernel uses several types of time-related devices, such as: real time clocks, time counters and event
       timers.   Real  time clocks are responsible for tracking real world time, mostly when the system is down.
       Time counters are responsible for tracking purposes, when  the  system  is  running.   Event  timers  are
       responsible  for  generating  interrupts at a 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 the kernel.  They provide monotonically increasing
       timestamps with known width and update frequency.  They can overflow, drift, etc and so in raw  form  can
       be used only in very limited performance-critical places like the 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 a unified way to access that hardware.

       Each driver implementing time counters registers them with the subsystem.  It is possible to see the list
       of present time counters, via the kern.timecounter sysctl(8) variable:

       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

       The output nodes are defined as follows:

       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 the quality of this time counter compared to
       others.  A negative value means this time counter is broken and should not be used.

       The  time  management  code of the kernel automatically switches to a higher-quality time counter when it
       registers, unless the kern.timecounter.hardware sysctl has been used to choose a specific device.

       There is no way to unregister a time counter once it has registered with the kernel.   If  a  dynamically
       loaded  module  contains  a  time  counter  you  will not be able to unload that module, even if the time
       counter it contains is not the one currently in use.

SEE ALSO

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

Debian                                           August 12, 2015                                 TIMECOUNTERS(4)