aio_connect ?

[Igor Sysoev]

On Wed, 20 Oct 2004, Christopher M. Sedore wrote:

> > > > While the developing my server nginx, I found the POSIX aio_*
> > > > operations
> > > > uncomfortable. I do not mean a different programming style, I mean
> > > > the aio_read() and aio_write() drawbacks - they have no
> > scatter-gather
> > > > capabilities (aio_readv/aio_writev) and they require too many
> > > > syscalls.
> > > > E.g, the reading requires
> > > > *) 3 syscalls for ready data: aio_read(), aio_error(),
> > aio_return()
> > > > *) 5 syscalls for non-ready data: aio_read(), aio_error(),
> > > >    waiting for notification, then aio_error(), aio_return(),
> > > >    or if timeout occuired - aio_cancel(), aio_error().
> > >
> > > This is why I added aio_waitcomplete().  It reduces both
> > cases to two
> > > syscalls.

Yes, aio_waitcomplete() can be used as the single waiting point. But then
I can not accept() connetions. How could I learn about the new connections ?

> > As I understand aio_waitcomplete() returns aiocb of any complete AIO
> > operation but I need to know the state of the exact AIO,
> > namely the last
> > aio_read().
>
> Correct, it won't poll, but what state can you get from calling
> aio_error() that you don't already know from aio_waitcomplete().  The
> operation has either completed (successfully or unsuccessfully) or it
> hasn't.  If it hasn't you haven't "gotten it back" via aio_waitcomplete,
> and if it has, you did.  I may be missing something, but how does
> aio_error() tell you something that you don't already know?

With aio_error() I may (and even have to) pass aiocb of the wanted operation.
aio_waitcomplete() returns aiocb of any operation. If I have several
operations there may be the race condition.

> > I use kqueue to get AIO notifications. If AIO operation would fail
> > at the start, will kqueue return notificaiton about this operation ?
>
> I don't think so--IIRC, if you have a parameter problem or the operation
> can't be queued, you'll get an error return from aio_read and no kqueue
> result. If it is queued, you'll get a kqueue notification.

Well, so I may not call aio_error() just after aio_read()/aio_write().

However, I can not use aio_waitcomplete() instead of aio_error()/aio_return()
pair after kevent() reports the completetion.


Igor Sysoev
http://sysoev.ru/en/