linux-kernel - Re: [patch] change futex

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Message-ID: <20070312112824.GA16093@elte.hu>
Date:	Mon, 12 Mar 2007 12:28:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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
Subject: Re: [patch] change futex_wait() to hrtimers


* Andi Kleen <andi@...stfloor.org> wrote:

> > if HIGH_RES_TIMERS is disabled then that is what happens. But 
> > frankly,
> 
> disabled? I would expect it (= more wakeups) when hrtimers are 
> enabled.

i mean the groupping of timer expiries happens automatically when 
high-res is disabled. When high-res is asked for then, duh, it's enabled 
and you get precise timeouts ;-)

> > most futex waits are without timeouts - if an application cares 
> > about micro-effects like that then you are much better off not using 
> > a per-futex timeout anywa
> 
> That sounds like you're arguing for not using hrtimers here because 
> the applications shouldn't depend on precise timeouts here anyways?!?

I was talking about the "micro-effect" of grouping timer expiries.

> Anyways when you convert more kernel timeouts to hrtimers you should 
> probably add some kind of batching facility that can be globally 
> configured with a sysctl or similar. Then at least laptops (and 
> possibly servers) can opt for more power saving again. For the futexes 
> alone it probably won't matter too much agreed, but I see a trend to 
> more hr.

yeah, we had that in earlier versions, it's trivial - nobody used it. So 
i'll wait for the actual measurements and a patch (or i can do the patch 
too, if someone comes up with the measurements). I've added 
/proc/timer_stats and /proc/timer_info for exactly such reasons.

( note that we've added the facility for even more imprecise sleeps to 
  the timer_list APIs - but for hrtimers it's a lot less clear-cut case. )

	Ingo
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