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SubjectRe: Is sendfile all that sexy?
On Wed, Jan 17, 2001 at 11:32:35AM -0800, Linus Torvalds wrote:
> In article <Pine.LNX.4.30.0101171454340.29536-100000@baphomet.bogo.bogus>,
> Ben Mansell <linux-kernel@slimyhorror.com> wrote:
> >
> >The current sendfile() has the limitation that it can't read data from
> >a socket. Would it be another 5-minute hack to remove this limitation, so
> >you could sendfile between sockets? Now _that_ would be sexy :)
>
> I don't think that would be all that sexy at all.
>
> You have to realize, that sendfile() is meant as an optimization, by
> being able to re-use the same buffers that act as the in-kernel page
> cache as buffers for sending data. So you avoid one copy.
>
> However, for socket->socket, we would not have such an advantage. A
> socket->socket sendfile() would not avoid any copies the way the
> networking is done today. That _may_ change, of course. But it might
> not. And I'd rather tell people using sendfile() that you get EINVAL if
> it isn't able to optimize the transfer..

Yes, socket->socket sendfile is not that sexy. I actually did this
for 2.2.16 in the obvious (and stupid) way, copying data into a buffer
and writing it it out again. The performance was unsurprisingly
_exactly_ identical to a userspace read()/write() loop.

There is a use for an optimized socket->socket transfer - proxying
high speed TCP connections. It would be exciting if the zerocopy
networking framework led to a decent socket->socket transfer.

-VAL
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