Command Section
TIMECOUNTERS(4)        FreeBSD Kernel Interfaces Manual        TIMECOUNTERS(4)

     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 second.

     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

     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

     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

     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)

FreeBSD 11.1-RELEASE-p4         August 12, 2015        FreeBSD 11.1-RELEASE-p4
Command Section