linux-kernel - Re: [GIT PULL] slab fixes for 3.2-rc4

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Date:	Wed, 4 Jan 2012 15:10:22 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, Pekka Enberg <penberg@...nel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] slab fixes for 3.2-rc4

On Wed, Jan 4, 2012 at 9:00 AM, Christoph Lameter <cl@...ux.com> wrote:
>
> xadd is 3 cycles. add is one cycle.

On some uarchs. On new uarchs it can be a single cycle, I think, and
on some uarchs it will even be microcoded and/or only go in one pipe
because it has that odd behavior that it writes both to memory and a
register, and thus doesn't fit the normal fastpaths.

The point is, xadd isn't actually any faster than just doing the
regular "add and read". It's *slower*.

There really isn't ever any reason to use xadd on percpu variables.
That's my point. You claimed that there was a performance advantage.
There really isn't.

So why are you still arguing?

                           Linus
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