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SubjectRe: When did High-Resolution Timers hit mainline?
With the suggestions from Bart and Thomas, I've now reworked the text
as below. Seem okay now?

Cheers,

Michael

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 determined by the value of the
kernel constant HZ.

The value of HZ varies across kernel versions and hardware
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, giv-
ing 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, respec-
tively, 0.01, 0.004, or 0.001 seconds. Since kernel 2.6.20, a
further frequency is available: 300, a number that divides
evenly for the common video frame rates (PAL, 25 HZ; NTSC, 30
HZ).

The times(2) system call is a special case. It reports times
with a granularity defined by the kernel constant USER_HZ.
Userspace applications can determine the value of this constant
using sysconf(_SC_CLK_TCK).

High-Resolution Timers
Before Linux 2.6.21, the accuracy of timer and sleep system
calls (see below) was also limited by the size of the jiffy.

Since Linux 2.6.21, Linux supports high-resolution timers
(HRTs), optionally configurable 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). You can determine whether
high-resolution timers are supported by checking the resolution
returned by a call to clock_getres(3) or looking at the
"resolution" entries in /proc/timer_list.

HRTs are not supported on all hardware architectures. (Support
is provided on x86, arm, and powerpc, among others.)
...
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).
==END==


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