linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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Date:	Mon, 25 Nov 2013 20:52:23 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <davidlohr@...com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	"Norton, Scott J" <scott.norton@...com>, tom.vaden@...com,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	Jason Low <jason.low2@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Mon, 25 Nov 2013, Davidlohr Bueso wrote:
> On Mon, 2013-11-25 at 18:32 +0100, Thomas Gleixner wrote:
> > If the smp_mb() is heavy weight, then it will hurt massivly in the
> > case where the hash bucket is not empty, because we add the price for
> > the smp_mb() just for no gain.
> > 
> > In that context it would also be helpful to measure the overhead on
> > x86 for the !empty case.
> 
> Absolutely, I will add these comparisons. If we do notice that we end up
> hurting the !empty case, would the current patch using atomic ops still
> be considered? We have made sure that none of the changes in this set
> affects performance on other workloads/smaller systems.

Please read my last reply to the atomic ops approach.

Aside of that we need numbers for a significant range of !x86.

Thanks,

	tglx
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