Igor Sysoev wrote:
On Sun, 17 Oct 2004, Ronald F. Guilmette wrote:
I'm sitting here looking at that man pages for aio_read and aio_write, and the question occurs to me: ``Home come there is no such thing as an aio_connect function?''
There are clearly cases in which one would like to perform reads
asynchronously, but likewise, there are cases where one might like
to also perform socket connects asynchronously. So how come no
aio_connect?
In FreeBSD you can do connect() on the non-blocking socket, then set the socket to a blocking mode, and post aio_read() or aio_write() operations on the socket.
FreeBSD allows to post AIO operaitons on non-connected socket. NT (and W2K, I believe) do not. This is why ConnectEx() appeared in XP. I do not know about other OSes, but I belive only FreeBSD and NT have the kernel level AIO sockets implementation without the threads emulation in the user level (Solaris) or without the quietly falling to synchronous behaviour (Linux).
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().
I think aio_* may be usefull for the zero-copy sockets, however, FreeBSD's aio_write() does not wait when the data would be acknowledged by peer and notifies the completion just after it pass the data to the network layer.
Now that we have real threads, it shuld be possible to write an aio library that is
implemented by having a bunch of underlying threads..
Igor Sysoev
http://sysoev.ru/en/
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