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)