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

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Message-Id: <20070807124009.9f6c2247.akpm@linux-foundation.org>
Date:	Tue, 7 Aug 2007 12:40:09 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jonathan Corbet <corbet@....net>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: [PATCH] msleep() with hrtimers

On Fri, 03 Aug 2007 12:37:12 -0600
Jonathan Corbet <corbet@....net> wrote:

> 
> Here's the second (and probably final) posting of the msleep() with
> hrtimers patch.  The problem being addressed here is that the current
> msleep() will stop for a minimum of two jiffies, meaning that, on a
> HZ=100 system, msleep(1) delays for for about 20ms.  In a driver with
> one such delay for each of 150 or so register setting operations, the
> extra time adds up to a few seconds.
> 
> This patch addresses the situation by using hrtimers.  On tickless
> systems with working timers, msleep(1) now sleeps for 1ms, even with
> HZ=100.
> 
> Most comments last time were favorable.  The one dissenter was Roman,
> who worries about the overhead of using hrtimers for this operation; my
> understanding is that he would rather see a really_msleep() function for
> those who actually want millisecond resolution.  I'm not sure how to
> characterize what the cost could be, but it can only be buried by the
> fact that every call sleeps for some number of milliseconds.  On my
> system, the several hundred total msleep() calls can't cause any real
> overhead, and almost all happen at initialization time.

I'd be surprised if there was significant overhead - the maximum frequency
at which msleep() can be called is 1000Hz.  We'd need an awful lot of
overhead for that to cause problems, surely?

<thinks he's missing something again>
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