linux-kernel - Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

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Message-ID: <20140131201401.GD2936@laptop.programming.kicks-ass.net>
Date:	Fri, 31 Jan 2014 21:14:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	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, 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.


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