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

Chris Mason
2009-01-14T02:58:19Z

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