Update: Debox sendfile modifications

[John-Mark Gurney]

Mike Silbersack wrote this message on Tue, Nov 04, 2003 at 11:02 -0600:
> Suppose that sendfile is called to send to a non-blocking socket, and that
> it detects that the page(s) required are not in memory, and that disk I/O
> will be necessary.  Instead of blocking, sendfile would call a sendfile
> helper kernel thread (either by calling kthread_create, or by having a
> preexisting pool.)  After dispatching this thread, sendfile would return
> EWOULDBLOCK to the caller.  Note that only a limited number of threads
> would exist (perhaps 8?), so, if all threads were busy, sendfile would
> have to block like it does at present.
> 
> Once the I/O was complete, the thread would call sowakeup (or whatever is
> called typically when a thread is now ready for writing) for the socket in
> question.  The application would call sendfile, like normal, but this time
> everything would succeed because the page would be in memory.
> 
> ---
> 
> If such a feature were implemented, it might have the same increased
> performance effect that your new return value does, except that it would
> require no modification for a non-blocking sendfile based application to
> take advantage of it.

This would not work with kqueues as kevents are level triggered, not
edge... so, this idea is bad/broken.  I don't even think it would work
with select since when you call select, it will check the availabilty
of writing at the select call time..  You'd have to add a flag to the
fd so mark it as delayed for another reason..  I don't think this feature
can be cleanly added w/o somehow tieing fd's together.

One call that I would like to see is a fd to fd copy.  You pass in both
fd's with maybe a length, and then a kernel helper thread does the copy
or something.  This would have other uses in that you can then "plumb"
pipes between programs in a more effecient manner...  This would also
possibly fix/replace sendfile as sendfile isn't more different.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."