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Comment by dietr1ch | Hacker News Reader

Anything that happens far enough from the CPU is async, and here far probably means 10cm thanks to the speed of light not being fast enough, even in vacuum.

So, any computation that spans a machine the size of our hands needs async unless you are willing to drop the clocks to push "far" a bit further away (and bring in power, heat and noise with it).

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Thanks for putting words into this.

Another cut off could be 3 cm away: the RAM. If data needs to go on the heap, be shared, one can consider the truth lives farther away than 3cm and thus has async/impure effects.

But the same way the CPU works very hard to hide that away from you (reordering, caches, etc), green threads could do the same with barely any performance hit.

Which is fine until we decide that actually, we _would_ like that performance back, and now we re-implement async flows at language level again, realise what an advantage it’s bets us, and then that trickles into other languages as they also seek those advantages.

With async, things happening far away can completely prevent you from making progress, in which case you are better off doing something else in the meantime, like simply blocking and scheduling a thread from a different process as even the cost of swapping threads, destroying the caches and other cpu state is minimal and nothing else can be done.

The optimization begins instead of a single thing prevents you from making progress, it's a set of things, so you can parallelize the tasks and handle them in the seemingly random order they happen to finish. Now the question is how is this not an event loop running async tasks?

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