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SubjectRe: When did High-Resolution Timers hit mainline?
On Tue, Jun 24, 2008 at 8:35 AM, Michael Kerrisk
<mtk.manpages@googlemail.com> wrote:
> Hi Ingo, Thomas,
>
> I want to update some timer and sleep man pages to reflect the arrival
> of high-resolution timers. However, it's not quite clear to me when
> HRTs properly arrived in mainline. Was it 2.6.21? And at that point,
> was the resolution for all timer and sleep calls based on HRTs, so
> that they all became more accurate? (Or was it the case that various
> system calls switched over to HRTs in different later kernel
> versions?) Specifically, I'm thinking of the following system calls
>
> nanosleep()
> clock_nanosleep()
> setitimer()
> timer_create()/timer_settime()
>
> Was it the case that pre 2.6.21 (or whatever) these were all
> jiffy-based in their accuracy, and then post 2.6.21, they were all HRT
> based (if CONFIG_HIGH_RES_TIMERS is enabled)?

So I got confused by the fact that CONFIG_HIGH_RES_TIMERS only
appeared in 2.6.21. I plan to include the following text to time(7)
for the next man-pages release. Does it look okay?

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).

High-Resolution Timers
Before Linux 2.6.16, the accuracy of timer and sleep system
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 CON-
FIG_HIGH_RES_TIMERS. On a system that supports HRTs, the accu-
racy 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).

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


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