Unfortunately that's only half-true. You can (and will) still get blockage sometimes with IOCP, it depends on a lot of factors, like how loaded the system is, I think. There is absolutely no guarantee that your I/O will actually occur asynchronously, only that you will be notified of its completion asynchronously.
Also, opening a file is also always synchronous, which is quite annoying if you're trying not to block e.g. a UI thread.
The implication of both of these is you still need dedicated I/O threads. I love IOCP as much as anyone, but it does have these flaws, and was very much designed to be used from multiple threads.
The only workaround I'm aware of was User-Mode Scheduling, which effectively notified you as soon as your thread got blocked, but it still required multiple logical threads, and Microsoft removed support for it in Windows 11.
Unfortunately that's only half-true. You can (and will) still get blockage sometimes with IOCP, it depends on a lot of factors, like how loaded the system is, I think. There is absolutely no guarantee that your I/O will actually occur asynchronously, only that you will be notified of its completion asynchronously.
Also, opening a file is also always synchronous, which is quite annoying if you're trying not to block e.g. a UI thread.
The implication of both of these is you still need dedicated I/O threads. I love IOCP as much as anyone, but it does have these flaws, and was very much designed to be used from multiple threads.
The only workaround I'm aware of was User-Mode Scheduling, which effectively notified you as soon as your thread got blocked, but it still required multiple logical threads, and Microsoft removed support for it in Windows 11.