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Date:	Fri, 31 Jan 2014 17:29:40 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, aswin@...com,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation

On Fri, 2014-01-31 at 16:09 -0500, Waiman Long wrote:
> On 01/31/2014 03:14 PM, Peter Zijlstra wrote:
> > On Fri, Jan 31, 2014 at 01:59:02PM -0500, Waiman Long wrote:
> >> On 01/31/2014 04:26 AM, Peter Zijlstra wrote:
> >>> On Thu, Jan 30, 2014 at 04:17:15PM +0100, Peter Zijlstra wrote:
> >>>> The below is still small and actually works.
> >>> OK, so having actually worked through the thing; I realized we can
> >>> actually do a version without MCS lock and instead use a ticket lock for
> >>> the waitqueue.
> >>>
> >>> This is both smaller (back to 8 bytes for the rwlock_t), and should be
> >>> faster under moderate contention for not having to touch extra
> >>> cachelines.
> >>>
> >>> Completely untested and with a rather crude generic ticket lock
> >>> implementation to illustrate the concept:
> >>>
> >> Using a ticket lock instead will have the same scalability problem as the
> >> ticket spinlock as all the waiting threads will spin on the lock cacheline
> >> causing a lot of cache bouncing traffic.
> > A much more important point for me is that a fair rwlock has a _much_
> > better worst case behaviour than the current mess. That's the reason I
> > was interested in the qrwlock thing. Not because it can run contended on
> > a 128 CPU system and be faster at being contended.
> >
> > If you contend a lock with 128 CPUs you need to go fix that code that
> > causes this abysmal behaviour in the first place.
> >

But the kernel should also be prepared for such situations, whenever
possible.

> >
> 
> I am not against the use of ticket spinlock as the queuing mechanism on 
> small systems. I do have  concern about the contended performance on 
> large NUMA systems which is my primary job responsibility. Depending on 
> the workload, contention can happens anywhere. So it is easier said than 
> done to fix whatever lock contention that may happen.
> 
> How about making the selection of MCS or ticket queuing either user 
> configurable or depending on the setting of NR_CPUS, NUMA, etc?

Users have no business making these decisions and being exposed to these
kind of internals. CONFIG_NUMA sounds reasonable to me.

Thanks,
Davidlohr

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