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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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