Provided by: freebsd-manpages_12.2-1_all bug


     timecounters — kernel time counters subsystem


     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


     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) 16777215 13467909 3579545 900 65535 62692 1193182 0 4294967295 3013495652 14318180 950 4294967295 4067509463 11458556 -100

     The output nodes are defined as follows: is a bitmask, defining valid counter bits, is a present counter value, is a counter update frequency, 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.


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