[dullgiulio]

The example of async I/O in Node.js is with a filesystem operation.

How does that work? AFAIK Linux, unlike windows, doesn't have a proper async API for filesystem I/O.

POSIX AIO is implemented using threads in libc and Linux AIO (io_submit, etc) only works with O_DIRECT (no kernel caching) and there are other issues depending on underlying filesystem driver.

Is there any solution?

[steveklabnik]

https://nikhilm.github.io/uvbook/filesystem.html

> The libuv filesystem operations are different from socket operations. Socket operations use the non-blocking operations provided by the operating system. Filesystem operations use blocking functions internally, but invoke these functions in a thread pool and notify watchers registered with the event loop when application interaction is required.

[bogomipz]

I'm not familiar with libuv but is there a dedicated "watcher" thread in a thread pool that the kernel wakes up when when the disk driver has completed that I/O that it was sleeping on? If so is there a dedicated "watcher" thread for each non-blocking I/O request handled by libuv?

[dullgiulio]

There is no watcher: threads get parked by the kernel as "not runnable" while they perform the I/O operation.

When the operation is finished, then they become runnable again so the kernel scheduler runs them--that is, it runs the code that comes after the I/O operation.

This code (from libuv) notifies the requester that the operation is complete.

[bogomipz]

Thanks. So is the libuv package implemented as two distinct components then consisting of:

1) The main event loop thread.

2) a thread pool of "workers" which make the actual I/O request and sleep while waiting for the I/O. Then when a "worker" thread is set to runnable again by the kernel(because it's I/O request has been completed)notifies the main event loop thread via a signal?

Is that correct?

[dullgiulio]

That's correct, except the notification back is probably via some queue (think of futures or Go channels) rather than signals.

[bogomipz]

I see that makes sense. Cheers.

[steveklabnik]

I’m not familiar at that level.

[biokoda]

Yes a user space thread pool which is what node.js does on top of libuv.