Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964896AbWBPUY0 (ORCPT ); Thu, 16 Feb 2006 15:24:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964900AbWBPUY0 (ORCPT ); Thu, 16 Feb 2006 15:24:26 -0500 Received: from zcars04f.nortel.com ([47.129.242.57]:25255 "EHLO zcars04f.nortel.com") by vger.kernel.org with ESMTP id S964896AbWBPUYZ (ORCPT ); Thu, 16 Feb 2006 15:24:25 -0500 Message-ID: <43F4DF54.6030303@nortel.com> Date: Thu, 16 Feb 2006 14:23:48 -0600 From: "Christopher Friesen" User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.6) Gecko/20040115 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Esben Nielsen CC: Arjan van de Ven , Daniel Walker , Ingo Molnar , linux-kernel@vger.kernel.org, Ulrich Drepper , Thomas Gleixner , Andrew Morton Subject: Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace? References: In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 16 Feb 2006 20:23:50.0374 (UTC) FILETIME=[E5D3A060:01C63336] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1099 Lines: 34 Esben Nielsen wrote: > On Thu, 16 Feb 2006, Arjan van de Ven wrote: > > >>On Thu, 2006-02-16 at 20:06 +0100, Esben Nielsen wrote: >>>Why does the list have to be in userspace? >> >>because it's faster ;) > Faster??? > As I see it, extra manipulations have to be done even in the non-congested > case: Every time the lock is taken the locking thread has to add the lock > to a the list, and reversely remove the lock from the list. I.e. > instructions are _added_ to the fast path where you stay purely in > userspace. > > I am ofcourse comparing to a solution where you do a syscall on everytime > you do a lock. The whole *point* of futexes is that on uncontested operations you don't have to do a syscall. Thus, if you can avoid taking a syscall while still getting reliability, you'll be faster. Dropping to kernelspace isn't free. Chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/