linux-kernel - Re: When did High-Resolution Timers hit mainline?

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Message-ID: <cfd18e0f0806250633j2901aa89u5609f1b9a4a37dc6@mail.gmail.com>
Date:	Wed, 25 Jun 2008 15:33:55 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Bart Van Assche" <bart.vanassche@...il.com>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	"Roman Zippel" <zippel@...ux-m68k.org>,
	"john stultz" <johnstul@...ibm.com>
Subject: Re: When did High-Resolution Timers hit mainline?

Hi Bart,

Just following up a little further here.

On Wed, Jun 25, 2008 at 8:32 AM, Bart Van Assche
<bart.vanassche@...il.com> wrote:
> On Wed, Jun 25, 2008 at 6:08 AM, Michael Kerrisk
> <mtk.manpages@...glemail.com> wrote:
>>   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.
>
> Maybe "size of a jiffy" should be replaced by "duration of a jiffy" ?
>
> An explanation of the impact of CONFIG_NO_HZ is missing.
>
> You also missed the fact that since the 2.6 kernel there are two
> constants related to time resolution, namely HZ and USER_HZ. HZ is the
> frequency of the timer interrupt, and 1/USER_HZ is the time resolution
> for system calls that use jiffies as time unit (e.g. the five values
> returned by the times() system call).

As far as I can tell, times() is the only system call that employs
USER_HZ.  Let me know if you think I'm wrong.  The only othe place
where USER_HZ seems to come into play is the time fields displayed in
/proc/PID/stat and /poc/stat.  (My point of verification here is
looking at usages of cputime_to_clock_t() and cputime64_to_clock_t()
in the kernel source.)

> The time resolution of e.g. the
> select() and poll() system calls is 1.0/HZ since the timeout for these
> system calls is specified as a struct timeval or struct timespec.

Yes.  So I think what I'll do is just add some text noting that the
times() syscall is a special case.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
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