Date: Fri, 31 Jan 2014 21:14:01 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
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
Message-ID: <20140131201401.GD2936@laptop.programming.kicks-ass.net>
References: <1390537731-45996-1-git-send-email-Waiman.Long@hp.com>
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 <20140130151715.GA5126@laptop.programming.kicks-ass.net>
 <20140131092616.GC5126@laptop.programming.kicks-ass.net>
 <52EBF276.1020505@hp.com>
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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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