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

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Date:	Mon, 2 Dec 2013 12:01:21 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <davidlohr@...com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...nel.org>,
	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 Sat, 30 Nov 2013, Davidlohr Bueso wrote:
> On Thu, 2013-11-28 at 12:59 +0100, Peter Zijlstra wrote:
> > On Wed, Nov 27, 2013 at 11:44:38PM -0800, Davidlohr Bueso wrote:
> > > How about both enlarging the table _and_ aligning the buckets? As you
> > > know, increasing the size of the table also benefits (particularly in
> > > larger systems) in having more spinlocks. So we reduce the amount of
> > > collisions and alleviate contention on the hb->lock. Btw, do you have
> > > any particular concerns about the larger hash table patch? 
> > 
> > My only concern was the amount of #ifdef.
> > 
> > Wouldn't something like the below also work?
> 
> Below are the results for a workload that stresses the uaddr hashing for
> large amounts of futexes (just make waits fail the uval check, so no
> list handing overhead) on an 80 core, 1Tb NUMA system.
> 
> +---------+--------------------+------------------------+-----------------------+-------------------------------+
> | threads | baseline (ops/sec) | aligned-only (ops/sec) | large table (ops/sec) | large table+aligned (ops/sec) |
> +---------+--------------------+------------------------+-----------------------+-------------------------------+
> |     512 |              32426 | 50531  (+55.8%)        | 255274  (+687.2%)     | 292553  (+802.2%)             |
> |     256 |              65360 | 99588  (+52.3%)        | 443563  (+578.6%)     | 508088  (+677.3%)             |
> |     128 |             125635 | 200075 (+59.2%)        | 742613  (+491.1%)     | 835452  (+564.9%)             |
> |      80 |             193559 | 323425 (+67.1%)        | 1028147 (+431.1%)     | 1130304 (+483.9%)             |
> |      64 |             247667 | 443740 (+79.1%)        | 997300  (+302.6%)     | 1145494 (+362.5%)             |
> |      32 |             628412 | 721401 (+14.7%)        | 965996  (+53.7%)      | 1122115 (+78.5%)              |
> +---------+--------------------+------------------------+-----------------------+-------------------------------+
> 
> Baseline of course sucks compared to any other performance boost, and we
> get the best throughput when applying both optimizations, no surprise.
> We do particularly well for more than 32 threads, and the 'aligned-only'
> column nicely exemplifies the benefits of SMP aligning the buckets
> without considering the reduction in collisions.

Right. So yes, we want both. And I fully agree with Peters dynamic
allocation approach.

Thanks,

	tglx
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