This is exactly what I came to the comments to point out. You're 100% right. If the operating system's scheduler is any good at all, it won't wake up a thread w

This is exactly what I came to the comments to point out. You're 100% right. If the operating system's scheduler is any good at all, it won't wake up a thread which is blocked on something like read or select system calls. NIO-based servers on the JVM will (in their idle state) have a bunch of operating system threads all blocked on a select system call, waiting for a connection on the TCP socket. They won't be woken up until there's something to do.