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

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Message-ID: <52EBF276.1020505@hp.com>
Date:	Fri, 31 Jan 2014 13:59:02 -0500
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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. That is the reason 
why I want to replace ticket spinlock with queue spinlock. If the 
16-byte size is an issue, I can use the same trick in the queue spinlock 
patch to reduce its size down to 8 bytes with a bit more overhead in the 
slowpath.

Another thing I want to discuss about is whether a bit more overhead in 
moderate contention cases is really such a bit deal. With moderate 
contention, I suppose the amount of time spent in the locking functions 
will be just a few percent at most for real workloads. It won't really 
be noticeable if the locking functions take, maybe, 50% more time to 
finish. Anyway, I am going to do more performance testing on low end 
machines.

-Longman
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