What happens when an I/O operation cannot be done asynchronously?
libuv uses its thread pool (default 4 threads) to run the operation off the main thread. This prevents blocking the event loop for operations that the OS cannot handle asynchronously, like some file system operations on certain platforms.
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More FAQs in How Async I/O Works in Node.js Through libuv
When JavaScript calls an async I/O operation, it is delegated to libuv, which handles it off the main thread using the OS's async capabilities or the thread pool. When complete, libuv queues the callback, and the event loop runs it on the main thread.
libuv handles the actual I/O operation off the main thread, using the OS's async capabilities (epoll, kqueue, IOCP) or the thread pool. When the operation completes, libuv queues the callback for the event loop to process.
The event loop processes callbacks in phases: timers, pending callbacks, poll, check, and close. When libuv queues a completed I/O callback, the event loop picks it up on its next poll phase and runs it on the main thread.
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