TIMECOUNTERS(4) 					   BSD 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 sys-
     tem 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 chooses one time counter from that list.  The current choice can be read and affected via the
     kern.timecounter.hardware tunable/sysctl.

SEE ALSO

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

BSD
								  April 12, 2014							       BSD