Man

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

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)

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