linux-kernel - Re: [PATCH -v9][RFC] mutex: implement adaptive spinning

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Date:	Wed, 14 Jan 2009 12:21:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH -v9][RFC] mutex: implement adaptive spinning


* Chris Mason <chris.mason@...cle.com> wrote:

> v10 is better that not spinning, but its in the 5-10% range.  So, I've 
> been trying to find ways to close the gap, just to understand exactly 
> where it is different.
> 
> If I take out:
> 	/*
> 	 * If there are pending waiters, join them.
> 	 */
> 	if (!list_empty(&lock->wait_list))
> 		break;
> 
> 
> v10 pops dbench 50 up to 1800MB/s.  The other tests soundly beat my 
> spinning and aren't less fair.  But clearly this isn't a good solution.

i think since we already decided that it's ok to be somewhat unfair (_all_ 
batching constructs introduce unfairness, so the question is never 'should 
we?' but 'by how much?'), we should just take this out and enjoy the speed 
...

	Ingo
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