Oh that's a really neat find. I'm not sure how 'instructions' map to 'reductions' in the sense that if you stop when a reduction is completed the system is in a fairly well defined state so you can switch context quickly, but when you stop in mid reduction you may have to save a lot more state. The neat thing about the BEAM is that it is effectively a perfect match for Erlang and any tricks like that will almost certainly come with some kind of price tag attached. An interrupt is super expensive compared to a BEAM context switch to another thread of execution, you don't see the kernel at all, it is the perfect balance between cooperative and preemptive multitasking. You can pretend it is the second but under the hood it is the first, the end result is lightning fast context switches.

But: great find, I wasn't aware of this at all and it is definitely an intriguing possibility.