linux-kernel - Re: [PATCH/RFC] msleep() with hrtimers

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Message-ID: <Pine.LNX.4.64.0707161601570.1818@scrub.home>
Date:	Mon, 16 Jul 2007 16:11:38 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Jonathan Corbet <corbet@....net>
cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH/RFC] msleep() with hrtimers

Hi,

On Sun, 15 Jul 2007, Jonathan Corbet wrote:

> The OLPC folks and I recently discovered something interesting: on a
> HZ=100 system, a call to msleep(1) will delay for about 20ms.  The
> combination of jiffies timekeeping and rounding up means that the
> minimum delay from msleep will be two jiffies, never less.  That led to
> multi-second delays in a driver which does a bunch of short msleep()
> calls and, in response, a change to mdelay(), which will come back in
> something closer to the requested time.
> 
> Here's another approach: a reimplementation of msleep() and
> msleep_interruptible() using hrtimers.  On a system without real
> hrtimers this code will at least drop down to single-jiffy delays much
> of the time (though not deterministically so).  On my x86_64 system with
> Thomas's hrtimer/dyntick patch applied, msleep(1) gives almost exactly
> what was asked for.

BTW there is another thing to consider. If you already run with hrtimer/ 
dyntick, there is not much reason to keep HZ at 100, so you could just 
increase HZ to get the same effect.

bye, Roman
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