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.Dd June 18, 2021
.Dt HZ 9
.Os
.Sh NAME
.Nm hz ,
.Nm tick ,
.Nm stathz ,
.Nm profhz
.Nd system time model
.Sh SYNOPSIS
.In sys/kernel.h
.Pp
.Vt extern int hz;
.Vt extern int tick;
.Vt extern int stathz;	/* deprecated */
.Vt extern int profhz;	/* deprecated */
.Sh DESCRIPTION
.Fx
utilizes periodic, one-shot, global or per-CPU
timing hardware using
.Xr eventtimers 9
to produce traditional clock behavior.
.Pp
The main clock is used to update the system's notion of time via
.Xr timecounters 9
and to pace periodic system callbacks via
.Xr callout 9 ,
.Xr epoch 9 ,
and other methods documented in
.Xr hardclock 9 .
That routine will be called approximately
.Va hz
times per second.
.Pp
The second clock, running at either
.Va stathz
or
.Va profhz
was used to gather timing statistics, but has been replaced with the more
functional
.Xr hwpmc 4 .
These values are returned for
.Qq compatibility
with
.Bx 4.4 ,
.St -p1003.1-2001
and the
.Xr setitimer 2
.Va ITIMER_PROF
flag, which were deprecated in
.St -p1003.1-2008
in favor of
.Xr timer_settime 2 .
.Pp
.Va tick
is the length of time in microseconds of one system tick.
.Pp
These system variables are also available as
.Em struct clockinfo
from
.Xr sysctl 3
and
.Sy kern.clockrate
from
.Xr sysctl 8 .
.Pp
The
.Va hz
rate may be overridden by defining
.Dv HZ
in the kernel configuration file or setting
.Sy kern.hz
system tuneable via
.Xr loader.conf 5 .
.Pp
The current default is 1000 Hz for a tick of 1 ms for real hardware.
For virtual machine guests, the default is 100 Hz for a tick of 10 ms.
Only override the default value if you really know what you are doing.
Due to the adaptive nature of timeouts, changing this value has less effect than
it had in the past.
.Sh SEE ALSO
.Xr setitimer 2 ,
.Xr timer_settime 2 ,
.Xr loader.conf 5 ,
.Xr callout 9 ,
.Xr eventtimers 9 ,
.Xr hardclock 9 ,
.Xr microtime 9 ,
.Xr time_second 9 ,
.Xr timecounters 9