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Comment by anonymoushn | Hacker News Reader

> You're confusing IO not happening because it's not needed with IO never happening. Just because a method can perform IO doesn't mean it actually does every time you call it. If I call async_read(N) for the next N bytes, that isn't necessarily going to touch the IO driver.

Maybe you can read the linked post again? The problem in the example in the post is that data keeps coming from the network. If you were to strace the program, you would see it calling read(2) repeatedly. The runtime chooses to starve all other tasks as long as these reads return more than 0 bytes. This is obviously not the only option available.

I apologize for charitably assuming that you were correct in the rest of my reply and attempting to fill in the necessary circumstances which would have made you correct

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Actually, no, I misread it trying to make sense of what you were posting so this post is edited.

This is just mundane non-blocking sockets. If the socket never needs to block, it won't yield. Why go through epoll/uring unless it returns EWOULDBLOCK?

For io_uring all the reads go through io_uring and generally don't send back a result until some data is ready. So you'll receive a single stream of syscall results in which the results for all fds are interleaved, and you won't even be able to write code that has one task doing I/O starving other tasks. For epoll, polling the epoll instance is how you get notified of the readiness for all the other fds too. But the important thing isn't to poll the socket that you know is ready, it's to yield to runtime at all, so that other tasks can be resumed. Amusingly upon reading the rest of the blog post I discovered that this is exactly what tokio does. It just always yields after a certain number of operations that could yield. It doesn't implement preemption.

Honestly I assumed you had read the article and were just confused about how tokio was pretending to have preemption. Now you reveal you hadn't read the article so now I'm confused about you in general, it seems like a waste of time. But I'm glad you're at least on the same page now, about how checking if something is ready and yielding to the runtime are separate things.

You're in a reply chain that began with another user claiming that tokio implements preemption by shooting signals at itself.

> But I'm glad you're at least on the same page now, about how checking if something is ready and yielding to the runtime are separate things.

I haven't ever said otherwise?

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