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Date:	Fri, 31 Jan 2014 17:29:40 -0800
From:	Davidlohr Bueso <>
To:	Waiman Long <>
Cc:	Peter Zijlstra <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>, Arnd Bergmann <>,,,, Steven Rostedt <>,
	Andrew Morton <>,
	Michel Lespinasse <>,
	Andi Kleen <>,
	Rik van Riel <>,
	"Paul E. McKenney" <>,
	Linus Torvalds <>,
	Raghavendra K T <>,
	George Spelvin <>,
	Tim Chen <>,,
	Scott J Norton <>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock

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

> >
> 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.


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