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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CA+55aFwgN4QGowU1+5NO0QWr4nT9KDyk8opiVgzmULJG-2eKzg@mail.gmail.com>
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/