linux-kernel - Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 7 Jan 2009 09:50:57 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <chris.mason@...cle.com>
cc:	Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, 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>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning



On Wed, 7 Jan 2009, Chris Mason wrote:
> 
> So far I haven't found any btrfs benchmarks where this is slower than
> mutexes without any spinning.  But, it isn't quite as fast as the btrfs
> spin.

Quite frankly, from our history with ext3 and other filesystems, using a 
mutex in the filesystem is generally the wrong thing to do anyway. 

Are you sure you can't just use a spinlock, and just release it over IO? 
The "have to do IO or extend the btree" case is usually pretty damn clear.

Because it really sounds like you're lock-limited, and you should just try 
to clean it up. A pure "just spinlock" in the hotpath is always going to 
be better.

		Linus
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