Date: 17 Mar 2001 18:18:28 +0100 From: Dag-Erling Smorgrav <des@ofug.org> To: Maxime Henrion <mux@qualys.com> Cc: arch@FreeBSD.org Subject: Re: Proposal for a new syscall Message-ID: <xzp4rwsco0r.fsf@flood.ping.uio.no> In-Reply-To: Maxime Henrion's message of "Sat, 17 Mar 2001 17:34:44 %2B0100" References: <20010317164411.A420@nebula.cybercable.fr> <xzpzoekcs3r.fsf@flood.ping.uio.no> <20010317173444.B420@nebula.cybercable.fr>
next in thread | previous in thread | raw e-mail | index | archive | help
Maxime Henrion <mux@qualys.com> writes: > Dag-Erling Smorgrav wrote: > > You don't seem to understand what funopen() really does... > I think I do, since I already used it successfully. Look at how it is > used in libfetch (AFAIK, you are the author of libfetch, I'm not sure > if you wrote this part though), Yes, I did. If you read that code you'll see it does a *lot* more than just pumping data from one file to another. > it's given a read function to read the > socket and a write function that will then write this into a file, and > this is used to download a file. This has absolutely nothing to do with zero-copy anything. The reader function doesn't need to be backed by a file - it can do *anything* it wants, even return a constant stream of "Maxime Henrion doesn't know what funopen() is for" :) > > > Having > > > such a syscall in the kernel would allow to implement "zero-copy" > > > wherever it is feasible. > > No. It would save you two copies and a bunch of syscalls, but it > > wouldn't be real zero-copy, just "n-2 copy" instead of "n copy". > And if n == 2 ? It's never the case. I think the best you can do in userland is n = 3, from a device to a file or socket, or from a file or socket to a device, by using mmap(2) on one side (can't do it on both - you have to mmap one device and write its contents to a file, or read from a file into an mmapped device). In the general case (file to file, file to socket, socket to file, socket to socket) the best you can do, even with mmap(), is n = 4. > > > Then, sendfile() would just be a particular case of this syscall, where > > > the input fd is a file and the output fd is a socket, and it could be > > > rewritten using it. > > No. Have you looked at the sendfile() code? > Probably not enough ; however I don't understand why it wouldn't be > possible to write a more generic function than sendfile() dealing with > any type of file descriptors that sendfile() could call then. It's not impossible, but it'd be a lot of work and it wouldn't be zero-copy. DES -- Dag-Erling Smorgrav - des@ofug.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?xzp4rwsco0r.fsf>