linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Fri, 09 Jan 2009 10:28:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, 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>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Thu, 2009-01-08 at 11:13 -0800, Linus Torvalds wrote:
> 
> On Thu, 8 Jan 2009, Chris Mason wrote:
> > 
> > It is less fair though, the 50 proc parallel creates had a much bigger
> > span between the first and last proc's exit time.  This isn't a huge
> > shock, I think it shows the hot path is closer to a real spin lock.
> 
> Actually, the real spin locks are now fair. We use ticket locks on x86.

> We _could_ certainly aim for using ticket locks for mutexes too, that 
> might be quite nice.

Not really, ticket locks cannot handle a spinner going away - and we
need that here.

I've googled around a bit and MCS locks
(http://www.cs.rice.edu/~johnmc/papers/asplos91.pdf) look like a viable
way to gain fairness in our situation.

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