Re: OT: why no file copy() libc/syscall ??

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Nov 13, 2003 at 12:22:14PM -0800, H. Peter Anvin wrote:
> Followup to:  <20031111085323.M8854@devserv.devel.redhat.com>
> By author:    Jakub Jelinek <jakub@redhat.com>
> In newsgroup: linux.dev.kernel
> > > 
> > > Actually, I think we should have a: 
> > > 
> > > 	long copy_fd_to_fd (int src, int dst, int len)
> > > 
> > > type of systemcall. 
> > 
> > We have one, sendfile(2).
> > 
> 
> It would be very nice if we could (a) expand the uses of sendfile(2),
> and (b) have the libc do the fallback to read/write/mmap as needed.

I actually hacked cp for a while and it improved cp some point percent
on normal machines.

See ftp://ftp.suse.com/pub/people/andrea/cp-sendfile/

the main downside and the reason it wasn't applied IIRC is the lack of
interruption of sendfile, basically for an huge file it would take a
while before C^c has any effect. The kernel isn't interrupting the
syscall.  This is no different from a huge read or write syscall (but
read/write are never huge or the buffer would need to be huge too, not
the case for sendfile that works zerocopy), so in theory we could
workaround it by entering/exiting kernel multiple times just to allow
the signal to be handled like in the read/write case.