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TIMECOUNTERS(4)		 BSD 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 coun-
     ters 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 sched-

     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 chooses one	time counter from that
     list.  The	current	choice can be read and affected	via the
     kern.timecounter.hardware tunable/sysctl.

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

BSD				April 12, 2014				   BSD


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