From owner-svn-src-user@FreeBSD.ORG Mon Jan 26 17:50:14 2009 Return-Path: Delivered-To: svn-src-user@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 57CB81065815 for ; Mon, 26 Jan 2009 17:50:14 +0000 (UTC) (envelope-from alc@cs.rice.edu) Received: from mail.cs.rice.edu (mail.cs.rice.edu [128.42.1.31]) by mx1.freebsd.org (Postfix) with ESMTP id 0E0948FC08 for ; Mon, 26 Jan 2009 17:50:12 +0000 (UTC) (envelope-from alc@cs.rice.edu) Received: from mail.cs.rice.edu (localhost.localdomain [127.0.0.1]) by mail.cs.rice.edu (Postfix) with ESMTP id 5BDAA2C2A81; Mon, 26 Jan 2009 11:29:05 -0600 (CST) X-Virus-Scanned: by amavis-2.4.0 at mail.cs.rice.edu Received: from mail.cs.rice.edu ([127.0.0.1]) by mail.cs.rice.edu (mail.cs.rice.edu [127.0.0.1]) (amavisd-new, port 10024) with LMTP id qWBXNDsTDWG9; Mon, 26 Jan 2009 11:28:57 -0600 (CST) Received: from adsl-216-63-78-18.dsl.hstntx.swbell.net (adsl-216-63-78-18.dsl.hstntx.swbell.net [216.63.78.18]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mail.cs.rice.edu (Postfix) with ESMTP id 521452C2A8E; Mon, 26 Jan 2009 11:28:57 -0600 (CST) Message-ID: <497DF2D8.6040004@cs.rice.edu> Date: Mon, 26 Jan 2009 11:28:56 -0600 From: Alan Cox User-Agent: Thunderbird 2.0.0.19 (X11/20090124) MIME-Version: 1.0 To: Jeff Roberson References: <200901260052.n0Q0qAWN031905@svn.freebsd.org> <20090125172049.R983@desktop> In-Reply-To: <20090125172049.R983@desktop> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Alan Cox , src-committers@freebsd.org, svn-src-user@freebsd.org Subject: Re: svn commit: r187701 - user/alc/zerocopy/sys/vm X-BeenThere: svn-src-user@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "SVN commit messages for the experimental " user" src tree" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 26 Jan 2009 17:50:18 -0000 Jeff Roberson wrote: > On Mon, 26 Jan 2009, Alan Cox wrote: > >> Author: alc >> Date: Mon Jan 26 00:52:09 2009 >> New Revision: 187701 >> URL: http://svn.freebsd.org/changeset/base/187701 >> >> Log: >> Retire the page-based copy-on-write mechanism. > Hey if we're doing this there are some mbuf fields I could axe too. :-) > > This isn't productive for anyone anyway eh? I've done page-flipping > to kernel for nfs and custom protocols but to userland it just doesn't > make sense without an api to support it. It's great for netperf. :-) More seriously, it works well for repeated transmission of the same immutable buffer. Otherwise, the overheads overwhelm the savings. For example, after transmission is complete, it requires a page fault on each page of the buffer to re-enable write access to the buffer. Need I say more ... That said, the real motivation for this work is that the page-based copy-on-write mechanism is still buggy. Specifically, it doesn't play nice with the buffer cache. I just don't see the point in investing any further effort in fixing it. Alan