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

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Date:	Wed, 25 Jun 2008 09:09:39 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Michael Kerrisk <mtk.manpages@...glemail.com>
cc:	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?

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.
 
>        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.

Thanks,

	tglx

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