From: "H. Peter Anvin" <hpa@zytor.com>
To: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)
Date: 8 Mar 2002 15:43:56 -0800 [thread overview]
Message-ID: <a6bibs$9mu$1@cesium.transmeta.com> (raw)
In-Reply-To: <E16jRAU-0007QU-00@the-village.bc.nu> <Pine.LNX.4.33.0203081252450.1412-100000@penguin.transmeta.com>
Followup to: <Pine.LNX.4.33.0203081252450.1412-100000@penguin.transmeta.com>
By author: Linus Torvalds <torvalds@transmeta.com>
In newsgroup: linux.dev.kernel
>
> On Fri, 8 Mar 2002, Alan Cox wrote:
> >
> > Can we go to cache line alignment - for an array of locks thats clearly
> > advantageous
>
> I disagree about the "clearly". Firstly, the cacheline alignment is CPU
> dependent, so on some CPU's it's 32 bytes (or even 16), on others it is
> 128 bytes.
>
> Secondly, a lot of locking is actually done inside a single thread, and
> false sharing doesn't happen much - so keeping the locks dense can be
> quite advantageous.
>
> The cases where false sharing _does_ happen and are a problem should be
> for the application writer to worry about, not for the kernel to force.
>
> So I think 8 bytes is plenty fine enough - with 16 bytes a remote
> possibility (I don't think it is needed, but it gives you som epadding for
> future expansion). And people who have arrays and find false sharing to be
> a problem can fix it themselves.
>
> I personally don't find arrays of locks very common. It's much more common
> to have arrays of data structures that _contain_ locks (eg things like
> having hash tables etc with a per-hashchain lock) and then those container
> structures may want to be cacheline aligned, but the locks themselves
> should not need to be.
>
Also, on UP this is all a waste.
-hpa
--
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt <amsp@zytor.com>
next prev parent reply other threads:[~2002-03-08 23:44 UTC|newest]
Thread overview: 95+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-03-05 7:01 [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) Rusty Russell
2002-03-05 21:23 ` Futexes III : performance numbers Hubertus Franke
2002-03-06 2:08 ` Rusty Russell
2002-03-06 14:28 ` Hubertus Franke
2002-03-06 17:23 ` [Lse-tech] " george anzinger
2002-03-07 0:25 ` Hubertus Franke
2002-03-07 0:35 ` Hubertus Franke
2002-03-05 22:39 ` [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) Davide Libenzi
2002-03-05 23:16 ` Hubertus Franke
2002-03-05 23:26 ` Davide Libenzi
2002-03-05 23:37 ` Peter Svensson
2002-03-05 23:50 ` Davide Libenzi
2002-03-08 0:07 ` Richard Henderson
2002-03-06 1:46 ` Rusty Russell
2002-03-06 2:03 ` Davide Libenzi
2002-03-06 7:54 ` Futexes III : performance numbers Rusty Russell
2002-03-06 14:46 ` Hubertus Franke
2002-03-06 16:13 ` Hubertus Franke
2002-03-06 20:36 ` Futexes V : Hubertus Franke
2002-03-07 4:21 ` Rusty Russell
2002-03-08 18:07 ` [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) Linus Torvalds
2002-03-08 19:03 ` Hubertus Franke
2002-03-08 19:22 ` Linus Torvalds
2002-03-08 20:29 ` Hubertus Franke
2002-03-08 20:48 ` Matthew Kirkwood
2002-03-08 21:02 ` Linus Torvalds
2002-03-08 23:15 ` Hubertus Franke
2002-03-08 23:36 ` Alan Cox
2002-03-08 23:41 ` Linus Torvalds
2002-03-08 23:56 ` Hubertus Franke
2002-03-09 2:12 ` Linus Torvalds
2002-03-11 14:14 ` Hubertus Franke
2002-03-09 0:03 ` H. Peter Anvin
2002-03-09 1:15 ` Alan Cox
2002-03-10 19:41 ` Linus Torvalds
2002-03-11 20:49 ` Pavel Machek
2002-03-10 19:58 ` Martin J. Bligh
2002-03-10 20:40 ` Alan Cox
2002-03-10 20:28 ` Martin J. Bligh
2002-03-10 21:05 ` Alan Cox
2002-03-13 7:40 ` Rusty Russell
2002-03-13 16:37 ` Alan Cox
2002-03-12 9:35 ` Helge Hafting
2002-03-08 20:40 ` Alan Cox
2002-03-08 20:57 ` Linus Torvalds
2002-03-08 23:43 ` H. Peter Anvin [this message]
2002-03-08 22:55 ` Hubertus Franke
2002-03-08 23:38 ` Alan Cox
2002-03-08 23:44 ` H. Peter Anvin
2002-03-08 20:47 ` george anzinger
2002-03-08 23:02 ` Hubertus Franke
2002-03-08 23:47 ` george anzinger
2002-03-09 1:11 ` Alan Cox
2002-03-09 1:20 ` Linus Torvalds
2002-03-09 4:49 ` Rusty Russell
2002-03-11 22:45 ` Linus Torvalds
2002-03-11 23:12 ` Hubertus Franke
2002-03-12 7:20 ` Rusty Russell
2002-03-12 14:56 ` Hubertus Franke
2002-03-13 4:02 ` Rusty Russell
2002-03-12 17:17 ` Linus Torvalds
2002-03-13 2:57 ` Rusty Russell
2002-03-09 4:51 ` Rusty Russell
2002-03-13 9:12 Martin Wirth
2002-03-13 19:41 ` Bill Davidsen
2002-03-13 19:52 ` Dave McCracken
2002-03-13 22:17 ` Bill Davidsen
2002-03-13 20:06 ` Alan Cox
2002-03-15 7:31 ` Rusty Russell
2002-03-15 8:41 ` Martin Wirth
2002-03-15 15:29 ` Hubertus Franke
2002-03-15 16:23 ` Peter Wächtler
2002-03-16 0:12 ` Rusty Russell
2002-03-16 11:23 ` Martin Wirth
2002-03-18 0:52 ` Ulrich Drepper
2002-03-19 3:28 ` Rusty Russell
2002-03-19 4:05 ` Ulrich Drepper
2002-03-20 6:20 ` Rusty Russell
2002-03-20 10:42 ` Peter Wächtler
2002-03-20 17:20 ` Ulrich Drepper
2002-03-19 8:34 ` Martin Wirth
2002-03-20 6:45 ` Rusty Russell
2002-03-21 6:48 ` Martin Wirth
2002-03-24 18:25 ` Peter Wächtler
2002-03-25 2:28 ` Rusty Russell
2002-03-25 4:46 ` Rusty Russell
2002-03-25 11:56 ` Peter Wächtler
2002-03-26 1:02 ` Rusty Russell
2002-03-26 8:17 ` Martin Wirth
2002-03-26 23:10 ` Rusty Russell
2002-03-27 21:05 ` Hubertus Franke
2002-03-27 23:53 ` Rusty Russell
2002-03-25 9:47 ` Peter Wächtler
2002-03-16 19:48 ` Peter Wächtler
2002-03-17 6:50 ` Rusty Russell
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