HZ(9)

HZ(9) | NetBSD Kernel Developer's Manual | HZ(9)

# NAME

hz, tick, tickadj, stathz, profhz — system time model

# SYNOPSIS

#include <sys/kernel.h>

extern int hz;
extern int tick;
extern int tickadj;
extern int stathz;
extern int profhz;

# DESCRIPTION

The essential clock handling routines in NetBSD are written to operate with two timers that run independently of each other. The main clock, running hz times per second, is used to keep track of real time.

In another words, hz specifies the number of times the hardclock(9) timer ticks per second. Normally hardclock(9) increments time by tick each time it is called. If the system clock has drifted, adjtime(2) may be used to skew this increment based on the rate of tickadj.

The second timer is used to gather timing statistics. It also handles kernel and user profiling. If the second timer is programmable, it is randomized to avoid aliasing between the two clocks. The mean frequency of the second timer is stathz. If a separate clock is not available, stathz is set to hz.

If profiling is enabled, the clock normally used to drive stathz may be run at a higher rate profhz, which is required to be a multiple of stathz. This will give higher resolution profiling information.

These system variables are also available as struct clockinfo from sysctl(3) and kern.clockrate from sysctl(8). The hz is hardware-dependent; it can be overridden (if the machine dependent code supports this) by defining HZ in the kernel configuration file (see options(4)). Only override the default value if you really know what you are doing.

# SEE ALSO

adjtime(2), callout(9), hardclock(9), microtime(9), time_second(9)

March 25, 2010 | NetBSD 5.99