Gregory's ServerAsync I/O is also often used for I/O multiplexing, which is when you have more than one source of I/O and want to have several I/O operations in flight at the same time, but otherwise you don't need the CPU while they work. You don't need async I/O for that, you need poll(2).
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6 comments
 @drewdevault I'd disagree here, a program can be I/O bound (let's say small requests to a slow service / on a slow network), but you might still want to e.g. accept input from the user while the slow service is doing its thing. If you don't have threads or async i/o, you're waiting on the network while the user thinks your program is broken.  @miki bzzzzzt, wrong You want I/O multiplexing to address this case, which is not the same thing as async I/O. @drewdevault how exactly does async I/O differ from I/O multiplexing in your understanding? I/O multiplexing is IIUC having a single thread of execution and using poll() / epoll() / select() / ... to periodically check which I/O is ready and then handling that (either directly or via callbacks). Most nontrivial programs that do that do it via callbacks and an event loop. 1/2  @drewdevault You don't need them for parallelism to utilize resources well. You might still want/need them for a simpler programming model. |
And finally, if your program is I/O bound then you don't need threads.