timecounters

Kernel uses several types of time-related devices, such as: real time clocks, time counters and event timers. Real time clocks responsible for tracking real world time, mostly when system is down. Time counters are responsible for tracking purposes, when system is running. Event timers are responsible for generating interrupts at 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 kernel. They provide monotonically increasing timestamps with known width and update frequency. They can overflow, drift, etc and so in raw form used only in very limited performance-critical places like 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 unified way to access that hardware.

Each driver implementing time counters, registers them at the subsystem. It is possible to see the list of present time counters, like this, via kern.timecounter sysctl:

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

where:

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 how good is this time counter, comparing to others. Negative value means that this time counter is broken and should not be used.

Time management code of the kernel chooses one time counter from that list. Current choice can be read and affected via kern.timecounter.hardware tunable/sysctl.

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