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

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Date:	Tue, 13 Jan 2009 21:58:19 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"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

On Tue, 2009-01-13 at 18:21 +0100, Peter Zijlstra wrote:
> On Tue, 2009-01-13 at 08:49 -0800, Linus Torvalds wrote:
> > 
> > So do a v10, and ask people to test.
> 
> ---
> Subject: mutex: implement adaptive spinning
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Mon Jan 12 14:01:47 CET 2009
> 
> Change mutex contention behaviour such that it will sometimes busy wait on
> acquisition - moving its behaviour closer to that of spinlocks.
> 

I've spent a bunch of time on this one, and noticed earlier today that I
still had bits of CONFIG_FTRACE compiling.  I wasn't actually tracing
anything, but it seems to have had a big performance hit.

The bad news is the simple spin got much much faster, dbench 50 coming
in at 1282MB/s instead of 580MB/s.  (other benchmarks give similar
results)

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 tried a few variations, like only checking the wait list once before
looping, which helps some.  Are there other suggestions on better tuning
options?

(I retested v7 and see similar results)

-chris

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