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

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Message-ID: <cfd18e0f0806250520u565d512as298f024df06da857@mail.gmail.com>
Date:	Wed, 25 Jun 2008 14:20:26 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Ingo Molnar" <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	"Roman Zippel" <zippel@...ux-m68k.org>,
	"john stultz" <johnstul@...ibm.com>,
	"Bart Van Assche" <bart.vanassche@...il.com>
Subject: Re: When did High-Resolution Timers hit mainline?

Thomas,

On Wed, Jun 25, 2008 at 9:09 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 25 Jun 2008, Michael Kerrisk wrote:
>>    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).
>
> Hmm, that's a bit backwards. We changed the internal handling of those
> interfaces to hrtimers in 2.6.16, but the accuracy is still jiffies
> unless you have CONFIG_HIGH_RES_TIMERS enabled (which is only possible
> as of 2.6.21) and your system provides the necessary hardware.

So then it's simply enough to say:

Since  Linux  2.6.17,  Linux  optionaly supports  high-resolution  timers
(configurable via CONFIG_HIGH_RES_TIMERS).

right?

>>        HRTs are not supported on all hardware architectures.  (Support
>>        is provided on x86, arm, and powerpc, among others.)
>
> Also you might point out that you can check whether high resolution
> timers are active via clock_getres() or by checking the resolution
> entry in /proc/timer_list.

Okay -- thanks.

Cheers,

Michael

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