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

[Chris Mason <chris.mason@oracle.com>]

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@chello.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