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30 .Nd kernel time counters subsystem
32 The kernel uses several types of time-related devices, such as: real time clocks,
33 time counters and event timers.
34 Real time clocks are responsible for tracking real world time, mostly when the system
36 Time counters are responsible for tracking purposes, when the system is running.
37 Event timers are responsible for generating interrupts at a specified time or
38 periodically, to run different time-based events.
39 This page is about the second.
41 Time counters are the lowest level of time tracking in the kernel.
42 They provide monotonically increasing timestamps with known width and
44 They can overflow, drift, etc and so in raw form can be used only in very limited
45 performance-critical places like the process scheduler.
47 More usable time is created by scaling the values read from the selected
48 time counter and combining it with some offset, regularly updated by
54 Different platforms provide different kinds of timer hardware.
55 The goal of the time counters subsystem is to provide a unified way to access
58 Each driver implementing time counters registers them with the subsystem.
59 It is possible to see the list of present time counters, via the
64 kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
65 kern.timecounter.tc.ACPI-fast.mask: 16777215
66 kern.timecounter.tc.ACPI-fast.counter: 13467909
67 kern.timecounter.tc.ACPI-fast.frequency: 3579545
68 kern.timecounter.tc.ACPI-fast.quality: 900
69 kern.timecounter.tc.i8254.mask: 65535
70 kern.timecounter.tc.i8254.counter: 62692
71 kern.timecounter.tc.i8254.frequency: 1193182
72 kern.timecounter.tc.i8254.quality: 0
73 kern.timecounter.tc.HPET.mask: 4294967295
74 kern.timecounter.tc.HPET.counter: 3013495652
75 kern.timecounter.tc.HPET.frequency: 14318180
76 kern.timecounter.tc.HPET.quality: 950
77 kern.timecounter.tc.TSC-low.mask: 4294967295
78 kern.timecounter.tc.TSC-low.counter: 4067509463
79 kern.timecounter.tc.TSC-low.frequency: 11458556
80 kern.timecounter.tc.TSC-low.quality: -100
83 The output nodes are defined as follows:
85 .It Va kern.timecounter.tc. Ns Ar X Ns Va .mask
86 is a bitmask, defining valid counter bits,
87 .It Va kern.timecounter.tc. Ns Ar X Ns Va .counter
88 is a present counter value,
89 .It Va kern.timecounter.tc. Ns Ar X Ns Va .frequency
90 is a counter update frequency,
91 .It Va kern.timecounter.tc. Ns Ar X Ns Va .quality
92 is an integral value, defining the quality of this time counter
94 A negative value means this time counter is broken and should not be used.
97 The time management code of the kernel automatically switches to a
98 higher-quality time counter when it registers, unless the
99 .Va kern.timecounter.hardware
100 sysctl has been used to choose a specific device.
102 There is no way to unregister a time counter once it has registered
104 If a dynamically loaded module contains a time counter you will not
105 be able to unload that module, even if the time counter it contains
106 is not the one currently in use.