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

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Message-ID: <CA+55aFxW70iKq5YT-icVuvsmH=xapMd5e+OnbL-8JbX7fD4M-Q@mail.gmail.com>
Date:	Fri, 22 Nov 2013 13:14:03 -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 12:35 PM, Waiman Long <waiman.long@...com> wrote:
>
> Yes, the extra latency of the fair lock in earlier patch is due to the need
> to do a second cmpxchg(). That can be avoided by doing a read first, but
> that is not good for good cache.  So I optimized it for the default unfair
> lock. By supporting only one version, there is no need to do a second
> cmpxchg anymore.

Ok, thanks.

> I reran the timing test on the 2.93GHz processor. The timing is the
> practically the same. I reused the old one for the 2.4GHz processor.

Sounds good. I just wanted to make sure the numbers were sane.

            Linus
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