linux-kernel - Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

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Message-ID: <CA+55aFzkqP84M7L6j__QHuks7DXTQnGqdCo3=a=u7qKdER9rEQ@mail.gmail.com>
Date:	Fri, 22 Nov 2013 11:14:15 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.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" <linux-arch@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.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>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

On Fri, Nov 22, 2013 at 11:04 AM, Waiman Long <Waiman.Long@...com> wrote:
>
> In term of single-thread performance (no contention), a 256K
> lock/unlock loop was run on a 2.4GHz and 2.93Ghz Westmere x86-64
> CPUs. The following table shows the average time (in ns) for a single
> lock/unlock sequence (including the looping and timing overhead):
>
> Lock Type                   2.4GHz      2.93GHz
> ---------                   ------      -------
> Ticket spinlock              14.9        12.3
> Read lock                    17.0        13.5
> Write lock                   17.0        13.5
> Queue read lock              16.0        13.4
> Queue write lock              9.2         7.8

Can you verify for me that you re-did those numbers? Because it used
to be that the fair queue write lock was slower than the numbers you
now quote..

Was the cost of the fair queue write lock purely in the extra
conditional testing for whether the lock was supposed to be fair or
not, and now that you dropped that, it's fast? If so, then that's an
extra argument for the old conditional fair/unfair being complete
garbage.

Alternatively, maybe you just took the old timings, and the above
numbers are for the old unfair code, and *not* for the actual patch
you sent out?

So please double-check and verify.

              Linus
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