From owner-freebsd-arch@FreeBSD.ORG Wed Jun 18 08:35:22 2003 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id B9F8337B401 for ; Wed, 18 Jun 2003 08:35:22 -0700 (PDT) Received: from critter.freebsd.dk (critter.freebsd.dk [212.242.86.163]) by mx1.FreeBSD.org (Postfix) with ESMTP id B413443F85 for ; Wed, 18 Jun 2003 08:35:21 -0700 (PDT) (envelope-from phk@phk.freebsd.dk) Received: from critter.freebsd.dk (localhost [127.0.0.1]) by critter.freebsd.dk (8.12.9/8.12.9) with ESMTP id h5IFZJBE041013 for ; Wed, 18 Jun 2003 17:35:20 +0200 (CEST) (envelope-from phk@phk.freebsd.dk) To: arch@freebsd.org From: Poul-Henning Kamp Date: Wed, 18 Jun 2003 17:35:19 +0200 Message-ID: <41012.1055950519@critter.freebsd.dk> Subject: userland access to devices is moving! X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 18 Jun 2003 15:35:23 -0000 I sat down and hacked up a simple prototype to test the concept I have been rambling about for some years: Going directly from filedescriptor to device driver thus bypassing the vnode, devfs and specfs layer. I implemented this for /dev/null and /dev/zero, and ran the simple benchmark "dd if=/dev/zero of=/dev/null count=1000000" Before: N 3 Average: 44.900752667 Stddev: 0.049906338 After: N 3 Average: 18.460190333 Stddev: 0.074019507 That is 26.4 microseconds saved for each read(2)+write(2) operation, or 41% improvement on my Athlon 700MHz machine. A bit more locking will probably be needed, so this will erode some of this number, but there will be something left I'm sure :-) The largest impact of this is that VOP_OPEN(), vn_open() and vn_open_cred() grows an argument (the fdesc index) which existing callers need to pass a -1, the rest is relatively local hacking in devfs and some adjustments in the descriptor code. I have overall found that the implementation of this is not as hard as I imagined, and if I doubted it before, I am now certain that this is the right way to go. I should have tried this long time ago... patch at: http://phk.freebsd.dk/patch/fdesc.patch -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.