trusty (4) timecounters.4freebsd.gz

NAME

     timecounters — kernel time counters subsystem

SYNOPSIS

     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.

DESCRIPTION

     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.

SEE ALSO

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