> I'm curious, what reason? In days gone by, processes who got to run on the single core that contemporary CPU had available, had to actively relinquish contro

> I'm curious, what reason? In days gone by, processes who got to run on the single core that contemporary CPU had available, had to actively relinquish control of the core back to the kernel. If a single process refused to do so, e.g. because the program hang, there was nothing the kernel could do about it, and the entire OS was blocked. The scheduler never ran, no other process would get CPU time, the whole thing was dead in the water, and all you could do was kick the "Reset" button (if present) or pull the power cord and reboot. Obviously, this is a very bad situation for an OS, which runs many processes from many sources. And because of that, we ditched this system, and went on to preemptive multitasking, where control is relinquished back to the scheduler after a time whether the process is okay with that or not. Async basically re-invented that system in userspace. We have an event loop, and we have processes that actively yield control to it. What happens if a subroutine refuses to do so? There is nothing the event loop can do about that. And it's really easy for this to happen. All it needs is a single synchronous call, say, to an external datastore, somewhere deep down in the callstack, and the awesome throughput of async goes bye bye.