“The hardest part of I/O isn’t reading data. It’s knowing when to stop waiting.”Every high-performance server eventually runs into the same problem: waiting. Not waiting for CPU, not waiting for memory, but waiting for the outside world. Network sockets sit idle. File descriptors stall. Thousands of connections exist, but only a few are active at any given moment.Early operating systems handled this poorly. Modern systems do not—largely because of mechanisms like epoll on Linux and kqueue on BSD...