linux-kernel - Re: [patch] change futex

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Date:	Mon, 12 Mar 2007 15:31:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Theodore Tso <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roland McGrath <roland@...hat.com>, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, npiggin@...e.de, drepper@...hat.com,
	oleg@...sign.ru, sebastien.dugue@...l.net,
	linux-kernel@...r.kernel.org
Cc:	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch] change futex_wait() to hrtimers


* Theodore Tso <tytso@....edu> wrote:

> What we probably need in the long-term, and not just for high 
> precision wakeups, is we need a way for waiters (either in the kernel 
> or in userspace) to specify a desired precision in their timers.  Is 
> it, "wake me up in a second, exactly", or "wake me up in a second, 
> plus or minus 10ms"?  (or 50ms?  or 100ms?).

such a facility exists already, see round_jiffies() and 
round_jiffies_relative(). There's some short blurb about it at:

  http://kernelnewbies.org/LinuxChanges#head-513ceda14f5d8cf5b8a7c81d7e3821543141ecb0

> This becomes especially important if we want the tickless code to 
> really shine as far as power management is concerned. [...]

yes. That's why we also implemented /proc/timer_stat, and this was 
measured and a few higher-frequency fuzzy waiters were converted to use 
round_jiffies(). Some other waiters were fixed in user-space. It's all 
dependent on actual measurements and circumstances.

	Ingo
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