Date: Thu, 5 Jul 2001 00:34:47 -0600 From: "Kenneth D. Merry" <ken@kdm.org> To: Mike Smith <msmith@FreeBSD.ORG> Cc: Kevin Hui <khui@cs.toronto.edu>, freebsd-questions@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG Subject: Re: Raw disk access in userland Message-ID: <20010705003447.A49002@panzer.kdm.org> In-Reply-To: <200107050642.f656gox00974@mass.dis.org>; from msmith@FreeBSD.ORG on Wed, Jul 04, 2001 at 11:42:50PM -0700 References: <20010704211233.A48146@panzer.kdm.org> <200107050642.f656gox00974@mass.dis.org>
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On Wed, Jul 04, 2001 at 23:42:50 -0700, Mike Smith wrote: > > 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. *hits head* I (of all people) should have remembered that. I wrote the zero copy stuff for the pass(4) driver (see cam_periph_mapmem() in sys/cam/cam_periph.c) using physio as a model. Ken -- Kenneth Merry ken@kdm.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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