linux-kernel - Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex

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Date:	Tue, 26 Nov 2013 07:38:27 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Scott Norton <scott.norton@...com>,
	Tom Vaden <tom.vaden@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()

On Tue, 2013-11-26 at 13:34 +0100, Thomas Gleixner wrote:
> On Tue, 26 Nov 2013, Peter Zijlstra wrote:
> 
> > On Tue, Nov 26, 2013 at 12:21:40PM +0100, Ingo Molnar wrote:
> > > I'm somewhat reluctant to chalk it up to a single mfence - maybe 
> > > timings/behavior changed in some substantial way?
> > 
> > Ah indeed! We also changed the case where an enqueueing futex sees the
> > uval change and bails. It is now far more expensive due to having to
> > both queue and unqueue, whereas before it wouldn't queue at all.
> > 
> > I suppose the idea was to offset that by not requiring locking on the
> > wake side.
> 
> Aside of that I really would be interrested in an explanation for the
> STDDEV going up by factor 5. That's a clear indicator for fishyness.

FWIW here's another run for the patched kernel, stddev went down to ~3,
yet the overhead is quite similar to original results:

Run summary [PID 17678]: blocking on 512 threads (at futex 0x60314c), waking up 1 at a time.

[Run 1]: Wokeup 512 of 512 threads (100.00%) in 14.0360 ms
[Run 2]: Wokeup 512 of 512 threads (100.00%) in 15.6660 ms
[Run 3]: Wokeup 512 of 512 threads (100.00%) in 13.9330 ms
[Run 4]: Wokeup 512 of 512 threads (100.00%) in 18.5790 ms
[Run 5]: Wokeup 512 of 512 threads (100.00%) in 14.1480 ms
[Run 6]: Wokeup 512 of 512 threads (100.00%) in 14.5930 ms
[Run 7]: Wokeup 512 of 512 threads (100.00%) in 13.4360 ms
[Run 8]: Wokeup 512 of 512 threads (100.00%) in 10.0730 ms
[Run 9]: Wokeup 512 of 512 threads (100.00%) in 16.9040 ms
[Run 10]: Wokeup 512 of 512 threads (100.00%) in 19.3800 ms

Wokeup 512 of 512 threads (100.00%) in 15.0700 ms

Thanks,
Davidlohr

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