Date: Wed, 04 Jul 2001 23:42:50 -0700 From: Mike Smith <msmith@freebsd.org> To: "Kenneth D. Merry" <ken@kdm.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: <200107050642.f656gox00974@mass.dis.org> In-Reply-To: Your message of "Wed, 04 Jul 2001 21:12:33 MDT." <20010704211233.A48146@panzer.kdm.org>
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> 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]
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