TIMECOUNTERS(4) FreeBSD Kernel Interfaces Manual TIMECOUNTERS(4)


timecounterskernel 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 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.


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.

April 12, 2014 FreeBSD