Re: hacking - aio_sendfile()

On Wed, Jul 10, 2013 at 04:36:23PM -0700, Adrian Chadd wrote:
> Hiya,
> 
> I've started writing an aio_sendfile() syscall.
> 
> http://people.freebsd.org/~adrian/ath/20130710-aio-sendfile-3.diff
> 
> Yes, the diff is against -HEAD and not stable/9.
> 
> It's totally horrible, hackish and likely bad. I've only done some
> very, very basic testing to ensure it actually works; i haven't at all
> stress tested it out yet. It's also very naive - I'm not at all doing
> any checks to see whether I can short-cut to do the aio there and
> then; I'm always queuing the sendfile() op through the worker threads.
> That's likely stupid and inefficient in a lot of cases, but it at
> least gets the syscall up and working.
Yes, it is naive, but for different reason.

The kern_sendfile() is synchronous function, it only completes after
the other end of the network communication allows it. This means
that calling kern_sendfile() from the aio thread blocks the thread
indefinitely by unbounded sleep.

Your implementation easily causes exhaustion of the aio thread pool,
blocking the whole aio subsystem. It is known that our aio does not work
for sockets for the same reason. I object against adding more code with
the same defect.

Proper route seems to rewrite aio for sockets using the upcalls.  The same
should be done for sendfile, if sendfile is given aio flavor.

> 
> I'd like some feedback and possibly some help in stress testing it to
> make sure it's functioning well.
> 
> Thanks,
> 
> 
> -adrian
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