bionic (4) timecounters.4freebsd.gz

NAME

     timecounters — kernel time counters subsystem

SYNOPSIS

     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.

DESCRIPTION

     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 automatically switches to a higher-quality time counter when it
     registers, unless the kern.timecounter.hardware sysctl has been used to choose a specific device.

     There is no way to unregister a time counter once it has registered with the kernel.  If a dynamically
     loaded module contains a time counter you will not be able to unload that module, even if the time counter
     it contains is not the one currently in use.

SEE ALSO

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