linux-kernel - Re: [tip:core/locking] x86/smp: Move waiting on contended ticket lock out of line

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Message-ID: <CA+55aFxHco9QKkzJshbae4athxGdCoZY0vBB3Y=mJ6deOLnJuQ@mail.gmail.com>
Date:	Wed, 27 Feb 2013 19:19:28 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	linux-tip-commits@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [tip:core/locking] x86/smp: Move waiting on contended ticket lock
 out of line

On Wed, Feb 27, 2013 at 6:58 PM, Rik van Riel <riel@...hat.com> wrote:
>
> On the other hand, both MCS and the fast queue locks
> implemented by Michel showed low variability and high
> performance.

On microbenchmarks, and when implemented for only one single subsystem, yes.

> The numbers for Michel's MCS and fast queue lock
> implementations appear to be both fast and stable.

I do think that doing specialized spinlocks for special areas may be a
rasonable approach, and it's quite possible that the SySV IPC thing is
one such area.

But no, I don't think the numbers I've seen for Michel's MCS are AT
ALL comparable to the generic spinlocks, and the interface makes them
incompatible as a replacement to even test in general.

Don't get me wrong: I think the targeted approach is *better*. I just
also happen to think that you spout big words about things that aren't
all that big, and try to make this a bigger deal than it is. The
benchmark numbers you point to are micro-benchmarks, and not all
comparable.

              Linus
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