From owner-freebsd-hackers Wed Jul 4 23:30: 5 2001 Delivered-To: freebsd-hackers@freebsd.org Received: from mass.dis.org (dhcp44-21.dis.org [216.240.44.21]) by hub.freebsd.org (Postfix) with ESMTP id 97F3C37B408; Wed, 4 Jul 2001 23:29:52 -0700 (PDT) (envelope-from msmith@mass.dis.org) Received: from mass.dis.org (localhost [127.0.0.1]) by mass.dis.org (8.11.4/8.11.3) with ESMTP id f656gox00974; Wed, 4 Jul 2001 23:42:51 -0700 (PDT) (envelope-from msmith@mass.dis.org) Message-Id: <200107050642.f656gox00974@mass.dis.org> X-Mailer: exmh version 2.1.1 10/15/1999 To: "Kenneth D. Merry" Cc: Kevin Hui , freebsd-questions@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG Subject: Re: Raw disk access in userland In-reply-to: Your message of "Wed, 04 Jul 2001 21:12:33 MDT." <20010704211233.A48146@panzer.kdm.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Wed, 04 Jul 2001 23:42:50 -0700 From: Mike Smith Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG > On Wed, Jul 04, 2001 at 20:18:51 -0400, Kevin Hui wrote: > > Then the question is whether the kernel is copying data between userspace > > and kernelspace or whether it just DMAs the data straight in/out of the > > user program's address space. In Linux raw-io, given that it is a block > > device and you are doing page-aligned block I/Os on it, is smart and does > > zero copies. While it may seem to be jumping through hoops, maybe it does > > have a performance advantage? > > The kernel copies between userland and the kernel. Er, no. See sys/kern/kern_physio.c for the details. -- ... every activity meets with opposition, everyone who acts has his rivals and unfortunately opponents also. But not because people want to be opponents, rather because the tasks and relationships force people to take different points of view. [Dr. Fritz Todt] V I C T O R Y N O T V E N G E A N C E To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message