timecounters(4) [freebsd man page]

TIMECOUNTERS(4) 					   BSD Kernel Interfaces Manual 					   TIMECOUNTERS(4)

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 sys-
     tem 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 chooses one time counter from that list.  The current choice can be read and affected via the
     kern.timecounter.hardware tunable/sysctl.

SEE ALSO

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

BSD
								  April 12, 2014							       BSD

Check Out this Related Man Page

MICROTIME(9)						   BSD Kernel Developer's Manual					      MICROTIME(9)

NAME

     bintime, getbintime, microtime, getmicrotime, nanotime, getnanotime -- get the current time

SYNOPSIS

     #include <sys/time.h>

     void
     bintime(struct bintime *bt);

     void
     getbintime(struct bintime *bt);

     void
     microtime(struct timeval *tv);

     void
     getmicrotime(struct timeval *tv);

     void
     nanotime(struct timespec *ts);

     void
     getnanotime(struct timespec *tsp);

DESCRIPTION

     The bintime() and getbintime() functions store the system time as a struct bintime at the addresses specified by bt.  The microtime() and
     getmicrotime() functions perform the same utility, but record the time as a struct timeval instead.  Similarly the nanotime() and
     getnanotime() functions store the time as a struct timespec.  The structures are described in timeval(3).

     The bintime(), microtime(), and nanotime() functions always query the timecounter to return the current time as precisely as possible.
     Whereas getbintime(), getmicrotime(), and getnanotime() functions are abstractions which return a less precise, but faster to obtain, time.

     The intent of the getbintime(), getmicrotime(), and getnanotime() functions is to enforce the user's preference for timer accuracy versus
     execution time.  They should be used where a precision of 1/HZ (e.g., 10 msec on a 100HZ machine, see hz(9)) is acceptable or where perfor-
     mance is priority.

     The system realtime clock is guaranteed to be monotonically increasing at all times.  As such, all calls to these functions are guaranteed to
     return a system time greater than or equal to the system time returned in any previous calls.  Comparable functions exist to retrieve the
     time elapsed since boot; see microuptime(9).

SEE ALSO

     settimeofday(2), bintime_add(9), inittodr(9), time_second(9), tvtohz(9)

CODE REFERENCES

     The implementation of the microtime() family of functions is in sys/kern/kern_tc.c as a part of the timecounter(9) framework.

     The implementation of the time counter sources used by the timecounter(9) is machine dependent, hence its location in the source code tree
     varies from architecture to architecture.

AUTHORS

     This manual page was written by Jeremy Cooper and
     Kelly Yancey <kbyanc@posi.net>.

BUGS

     Despite the guarantee that the system realtime clock will always be monotonically increasing, it is always possible for the system clock to
     be manually reset by the system administrator to any date.

BSD
								   June 8, 2010 							       BSD

Linux and UNIX Man Pages

timecounters(4) [freebsd man page]

Check Out this Related Man Page