In cooperative multitasking you can program when to give up control, not when it is regained. The regaining part is unpredictable. Which introduces a lot of non-determinism to deal with and overhead.

This is no different than async/await. At some point you await a scheduled primitive, it could be a timer, io readiness, an io completion... and yield to a scheduler. You don’t specify explicitly when you return. These are not tightly coupled coroutines. This is precisely what is going on in cooperative multitasking.

I don’t see how this increases overhead to deal with either.

Basically, coop multitasking and async/await operate on the exact same execution framework, the latter just gives convenient syntactic support.

Perhaps you should see how typescript turns async await into js.

Await is just syntactic sugar. You do not really await anything. What actually happens is an event handler gets called on an event, where it sets up more event handlers for more events and so on. This is the essence of asynchronous programming. There are no tasks, no yielding, practically no overhead and everything is deterministic (in relation to external events obviously) [1]. The only cooperative multitasking implementations that have the same amount of determinism are those implemented strictly on top of event loops and that lack yielding function, so they cannot really be called cooperative multitasking implementations, as they can't "cooperate". All actual implementations have yielding, do not get control deterministically (dealing with that non-determinism requires stuff like semaphores) and have relatively significant overhead.

[1] If implemented with care, not doing syscalls in the middle of async primitives and using fast nearly-O(1) algorithms for timers, etc. it can be incredibly fast. And of course Rust also gives enough room to mess up all that nice determinism.

> You do not really await anything. What actually happens is an event handler gets called on an event,

So the event handler gets called immediately? No that’s not right. What would be the point of that? The event handler or continuation obviously needs to be scheduled on something that is awaitable. Meanwhile, other concurrent tasks may be able to run.

> This is the essence of asynchronous programming. There are no tasks, no yielding, practically no overhead and everything is deterministic

This is just totally wrong. Especially re tasks: https://docs.python.org/3/library/asyncio-task.html#creating...

There is nothing inherent about async and await that prevents “yielding”... the issue of yielding and semaphores is a concurrency issue and since async and await are used in concurrent programming environments, the same issues apply.

While it is true async and await don’t require any kind of cooperative concurrent framework to work, that is kind of their whole point for existing. A single task async/await system isn’t terribly interesting.

> So the event handler gets called immediately? No that’s not right. What would be the point of that? The event handler or continuation obviously needs to be scheduled on something that is awaitable. Meanwhile, other concurrent tasks may be able to run.

It's kind of like this: async/await is syntactic sugar for higher-order abstractions around event loops. At the level of event loops and event hadnlers there is no awaiting anymore. And the whole point of event loops is to not run event handlers concurrently, that's why they are even called loops, they invoke handlers one by one in a loop deterministically without concurrent tasks and once there is nothing more to run they just block and wait for new events. Obviously you can run multiple event loops in parallel, but you shouldn't share memory between them, as it defeats the purpose, is always slower and is never really necessary, you can just use asynchronous message passing to communicate between event loops when you have to.

> A single task async/await system isn’t terribly interesting.

And yet this is the whole point of async/await, promises, futures and event loops. All of them exist to avoid mistakes and performance problems of shared memory concurrency. I mean, really, if you have semaphores or mutexes in event handlers, futures, promises or async functions - you are in a broken concurrency model zone.

It is regained in exactly the same cases it would be in the async model: when a blocking operation completes and the scheduler resumes the now ready thread. As scheduler is called executor in the async world, while a thread is a coroutines, but the concepts are very similar.