Okay -- as a first step to resolving this, I've adjusted the text in
time.7, including adding a mention of HRTs. The text that I plan to
put in man-pages-3.01 is shown below. Does it look okay to you
Stephane?
The Software Clock, HZ, and Jiffies
The accuracy of various system calls that set timeouts,
(e.g., select(2), sigtimedwait(2)) and measure CPU time
(e.g., getrusage(2)) is limited by the resolution of the
software clock, a clock maintained by the kernel which
measures time in jiffies. The size of a jiffy is deter-
mined by the value of the kernel constant HZ.
The value of HZ varies across kernel versions and hard-
ware platforms. On i386 the situation is as follows: on
kernels up to and including 2.4.x, HZ was 100, giving a
jiffy value of 0.01 seconds; starting with 2.6.0, HZ was
raised to 1000, giving a jiffy of 0.001 seconds. Since
kernel 2.6.13, the HZ value is a kernel configuration
parameter and can be 100, 250 (the default) or 1000,
yielding a jiffies value of, respectively, 0.01, 0.004,
or 0.001 seconds. Since kernel 2.6.20, a further fre-
quency is available: 300, a number that divides evenly
for the common video frame rates (PAL, 25 HZ; NTSC, 30
HZ).
High-Resolution Timers
Before Linux 2.6.16, the accuracy of timer and sleep sys-
tem calls (see below) was also limited by the size of the
jiffy.
Since Linux 2.6.16, Linux supports high-resolution timers
(HRTs), optionally configurable since kernel 2.6.21 via
CONFIG_HIGH_RES_TIMERS. On a system that supports HRTs,
the accuracy of sleep and timer system calls is no longer
constrained by the jiffy, but instead can be as accurate
as the hardware allows (microsecond accuracy is typical
of modern hardware).
...
Sleeping and Setting Timers
Various system calls and functions allow a program to
sleep (suspend execution) for a specified period of time;
see nanosleep(2), clock_nanosleep(2), and sleep(3).
Various system calls allow a process to set a timer that
expires at some point in the future, and optionally at
repeated intervals; see alarm(2), getitimer(2),
timerfd_create(2), and timer_create(3).
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