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From: Waiman Long <waiman.long@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arch@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michel Lespinasse <walken@google.com>,
	Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	George Spelvin <linux@horizon.com>,
	Tim Chen <tim.c.chen@linux.intel.com>, "" <aswin@hp.com>,
	Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation
Date: Fri, 31 Jan 2014 13:59:02 -0500	[thread overview]
Message-ID: <52EBF276.1020505@hp.com> (raw)
In-Reply-To: <20140131092616.GC5126@laptop.programming.kicks-ass.net>

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

  parent reply	other threads:[~2014-01-31 18:59 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-01-24  4:28 [PATCH v11 0/4] Introducing a queue read/write lock implementation Waiman Long
2014-01-24  4:28 ` [PATCH v11 1/4] qrwlock: A " Waiman Long
2014-01-24  8:25   ` Peter Zijlstra
2014-01-25  4:39     ` Waiman Long
2014-01-24  4:28 ` [PATCH v11 2/4] qrwlock, x86: Enable x86 to use queue read/write lock Waiman Long
2014-01-24  4:28 ` [PATCH v11 3/4] qrwlock, x86: Add char and short as atomic data type in x86 Waiman Long
2014-01-24  4:28 ` [PATCH v11 4/4] qrwlock: Use the mcs_spinlock helper functions for MCS queuing Waiman Long
2014-01-24  8:26   ` Peter Zijlstra
2014-01-25  4:30     ` Waiman Long
2014-01-30 13:04 ` [PATCH v11 0/4] Introducing a queue read/write lock implementation Peter Zijlstra
2014-01-30 15:17   ` Peter Zijlstra
2014-01-30 15:43     ` Waiman Long
2014-01-30 15:50       ` Waiman Long
2014-01-30 15:53         ` Peter Zijlstra
2014-01-30 15:44     ` Peter Zijlstra
2014-01-30 17:52       ` Will Deacon
2014-01-30 18:05         ` Peter Zijlstra
2014-01-30 18:11           ` Will Deacon
2014-01-30 18:16             ` Peter Zijlstra
2014-01-31  9:26     ` Peter Zijlstra
2014-01-31 10:03       ` George Spelvin
2014-01-31 10:17         ` Peter Zijlstra
2014-01-31 11:30           ` Peter Zijlstra
2014-02-01 23:22           ` Paul E. McKenney
2014-01-31 18:59       ` Waiman Long [this message]
2014-01-31 19:47         ` Peter Zijlstra
2014-02-01 10:38           ` George Spelvin
2014-01-31 20:14         ` Peter Zijlstra
2014-01-31 21:09           ` Waiman Long
2014-02-01  1:29             ` Davidlohr Bueso
2014-02-02  9:03             ` Ingo Molnar
2014-02-03 11:38               ` Peter Zijlstra
2014-02-06  3:08               ` Waiman Long

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