Subject: Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning
From: Chris Mason <chris.mason@oracle.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
       Gregory Haskins <ghaskins@novell.com>, Ingo Molnar <mingo@elte.hu>,
       Matthew Wilcox <matthew@wil.cx>, Andi Kleen <andi@firstfloor.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       linux-btrfs <linux-btrfs@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Steven Rostedt <rostedt@goodmis.org>, Nick Piggin <npiggin@suse.de>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>
In-Reply-To: <alpine.LFD.2.00.0901070945450.3057@localhost.localdomain>
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Date: Wed, 07 Jan 2009 13:00:12 -0500
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On Wed, 2009-01-07 at 09:50 -0800, Linus Torvalds wrote:
> 
> 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.

There are definitely ways I can improve performance for contention in
the hot btree nodes, and I think it would be a mistake to tune the
generic adaptive locks just for my current code.

But, it isn't a bad test case to compare the spin with the new patch and
with the plain mutex.  If the adaptive code gets in, I think it would be
best for me to drop the spin.

Either way there's more work to be done in the btrfs locking code.

-chris


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