.\"	$OpenBSD: hz.9,v 1.9 2019/01/14 02:05:46 tedu Exp $
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.\" Copyright (c) 1999 Marc Espie
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.Dd $Mdocdate: January 14 2019 $
.Dt HZ 9
.Os
.Sh NAME
.Nm hz ,
.Nm tick ,
.Nm tickadj ,
.Nm stathz ,
.Nm profhz
.Nd system time model
.Sh SYNOPSIS
.Vt extern int hz;
.Vt extern int tick;
.Vt extern int tickadj;
.Vt extern int stathz;
.Vt extern int profhz;
.Sh DESCRIPTION
The system is driven by
.Xr hardclock 9
interrupts, which occur at
.Va hz
frequency, and are used to keep track of real time.
.Pp
On systems where another independent clock is available, it is set at
.Va stathz
frequency, and used to gather timing statistics.
Ideally, it would be better to drive
.Va stathz
with a slightly randomized clock, that is still a fixed number on average,
as this would prevent malicious processes from working around the scheduler.
If a separate clock is not available,
.Va stathz
is set to
.Va hz .
.Pp
If profiling is enabled, the clock normally used to drive
.Va stathz
may be run at a higher rate
.Va profhz ,
which must be a multiple of
.Va stathz .
This will give higher resolution profiling information.
.Pp
Normally,
.Xr hardclock 9
increments
.Va time
by
.Va tick
each time it is called.
If the system clock has drifted,
.Xr adjtime 2
may be used to skew this increment, but by no more
than ten times
.Va tickadj .
.Pp
These system variables are available by reading
.Dv KERN_CLOCKRATE
from
.Xr sysctl 2 .
.Sh SEE ALSO
.Xr adjtime 2 ,
.Xr clock_getres 2 ,
.Xr sysctl 2 ,
.Xr hardclock 9 ,
.Xr microtime 9