Re: Is sendfile all that sexy?

[Val Henson]

On Wed, Jan 17, 2001 at 11:32:35AM -0800, Linus Torvalds wrote:
> In article <[e-mail]>,
> Ben Mansell  <[e-mail]> 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