man/man9/hz.9

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.Dd July 1, 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;
.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.
These clocks regulate periodic events in the system.
.Pp
The main clock is used to update the system's notion of time via
.Xr timecounters 9
and to pace periodic system processing as documented in
.Xr hardclock 9 .
That routine may be called once every 1 /
.Va hz
seconds, though the call is omitted if no work is needed in the next tick and it
has not been 0.5 seconds since the last call.
.Pp
The stat clock running at
.Va stathz
gathers statistics on the system and its processes.
It computes values for
.Xr getrusage 2
and statistics displayed by
.Xr ps 1
and
.Xr top 1 .
.Pp
Finally, a profiling clock may run at
.Vt profhz
to sample user program counter values for profiling purposes.
This profiling mechanism has been replaced by the more functional
.Xr hwpmc 4
and may be removed in a future version of
.Fx .
.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 current global and per-CPU CPU time usage is returned to the user in units
of 1 /
.Va stathz
ticks in the
.Sy kern.cp_time
and
.Sy kern.cp_times
sysctl MIBs.
.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 ps 1 ,
.Xr top 1 ,
.Xr setitimer 2 ,
.Xr timer_settime 2 ,
.Xr loader.conf 5 ,
.Xr eventtimers 9 ,
.Xr hardclock 9 ,
.Xr microtime 9 ,
.Xr time_second 9 ,
.Xr timecounters 9
.Sh IMPLEMENTATION NOTES
Historically, both the
.Va stathz
and
.Va profhz
clocks have run off the same physical timer running at the slower rate when no
process is using the profile features, or at the higher rate when at least one
process is using it.
Although the interface is deprecated by
.St -p1003.1-2008
in favor of
.Xr timer_settime 2 ,
several programs still use
.Xr setitimer 2
and
.Va ITIMER_PROF
for a higher-resolution periodic interrupt than has been traditionally
available.
.Pp
Historically,
.Xr hardclock 9
has also been run off a separate interrupt, except on constrained platforms that
lack enough periodic interrupt sources.
.Fx
uses
.Xr eventtimers 9
to abstract these details away, though some old code may still harbor
assumptions that are an imperfect fit to this abstraction.
.Pp
.Xr timecounters 9
are limited to 32-bits and wrap after about a second, so we must update the
time hands they maintain at least every half second to get the proper wrapping
math.
In addition,
.Va kern.cp_times
needs to updated at least once a second so that the values displayed by
.Xr top 1
update every second.