From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265301AbUGMPCs (ORCPT ); Tue, 13 Jul 2004 11:02:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265312AbUGMPCs (ORCPT ); Tue, 13 Jul 2004 11:02:48 -0400 Received: from gort.metaparadigm.com ([203.117.131.12]:61346 "EHLO gort.metaparadigm.com") by vger.kernel.org with ESMTP id S265301AbUGMPCq (ORCPT ); Tue, 13 Jul 2004 11:02:46 -0400 Message-ID: <40F3F993.6040602@metaparadigm.com> Date: Tue, 13 Jul 2004 23:02:43 +0800 From: Michael Clark Organization: Metaparadigm Pte Ltd User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7) Gecko/20040707 Debian/1.7-5 X-Accept-Language: en MIME-Version: 1.0 To: Lutz Vieweg Cc: Robin Holt , linux-kernel@vger.kernel.org Subject: Re: How to find out which pages were copied-on-write? References: <40EACC0C.6060606@isg.de> <20040709113125.GA8897@lnx-holt.americas.sgi.com> <40EF0346.4040407@isg.de> <40EFA4C8.1050409@metaparadigm.com> <40F2C882.7070406@isg.de> <40F36216.1080603@metaparadigm.com> <40F3DDC4.7060104@isg.de> In-Reply-To: <40F3DDC4.7060104@isg.de> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 07/13/04 21:04, Lutz Vieweg wrote: >> You don't use mmap for speed but rather for convenience. > > > But isn't an advantage with mmap() that there's no need for the kernel > to copy what is to be written to a dedicated buffer? The kernel > could initiate DMA writes directly from the working memory... Yes, but page faults are expensive too. Each time a page is written out it needs to be marked read only again and will cause a page fault for the next write access from userspace. For certain workloads this can easily add up to more than copy_(to|from)_user in read/write. read/write also gives you more explicit control on IO batching and scheduling (when to read or write). Less need for the kernel to employ tricks to effectively coaslesce IOs on dirtied pages or sense streaming access patterns. ~mc