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.