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graetzer | 5 years ago

Probabbly don't really know what I am talking about, since only ever used async IO through boost.asio:

Isn't it a bit crazy that there are so many different ways of doing async IO on linux alone ? You would think that this is a more or less solved problem by now.

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kccqzy|5 years ago

No, I don't think so.

The terminology here is a bit confusing because different people have defined "async" differently, but in the strictest sense of the word, before io_uring the only other way to do async IO is POSIX AIO. See aio(7) in your man pages: http://man7.org/linux/man-pages/man7/aio.7.html

On the other hand, using select(2), poll(2), and epoll(2) is not really async IO.

The difference is really simple: for non-async IO, whenever you use read(2) or write(2) and that call returns without an errno, that operation is decidedly complete from the perspective of the user space. The buffer might not actually make it to disk (say because of caching) or the network (say because of the Nagle algorithm). But really from the user space perspective it is done.

What about "async" libraries in user space? All the bells and whistles added by these libraries in user space merely give you support for knowing when a file descriptor is ready. It doesn't make the actual reading or writing asynchronous. So with a such library, your code might appear to call read(), but the framework knows that the file descriptor isn't actually ready, suspends your greenthread/fiber/coroutine or whatever it uses and does something else. The actual read(2) call doesn't happen. The kernel doesn't know you want to read.

With io_uring, you actually deal with read or write requests sent to the kernel that are incomplete. You actually tell the kernel you want to read (by writing to a ring buffer, hence the name). The kernel knows you want to read. That's the difference.

Now don't get me wrong but I think for the majority of applications you don't need true async IO. All you really need is efficient notification of whether a file descriptor is ready. So AIO and io_uring are both niche topics that likely won't affect your app.

loeg|5 years ago

Really importantly, the behavior of poll/select/epoll on files is to always return "available." But a read from that file may still sleep waiting on disk — available does not mean the result is already in memory. They're only really effective primitives for networking sockets and artificial fds like signalfd().

Prior to io_uring, libraries that provide async file access in userspace (by necessity) use some kind of threadpool, with each thread processing an operation synchronously and providing an async result via self-pipe or other user-driven event.

gpderetta|5 years ago

Both competition notification and readiness notification are forms of async io. Buffering in userspace instead in kernel space is not a meaningful difference. Calling the latter not true asynch io seems incorrect to me.

It is true that readiness notification does not lend well to disk io though.