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Comment by jellyksong | Hacker News Reader

Basic question: Why is this faster than running Intel Linux apps in an emulated Intel Linux VM? Because Rosetta is faster than QEMU, and you only need to emulate one application rather than the entire kernel?

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Emulation of an x86 kernel level means you lose the hardware-assisted virtualization support you'd get with an ARM kernel, and emulating an MMU is slow (among other things.)

Technically this would be replacing QEMU user-mode emulation. Which isn't fast in a large part because QEMU being portable to all host architectures was more important than speed.

a lot of the performance gains in rosetta 2 come from load time translation of executables and libraries. so when you run a binary on the host mac, rosetta jumps in, does a one time translation to something that can run natively on the mx processor and then (probably) caches the result on the filesystem. next time you run it, it runs the cached translation. if you're running a vm without support inside the guest for this, then you're just running a big intel blob on the mx processor and having to do realtime translation which is really slow. (worst case, you have an interrupt for every instruction, as you have to translate it... although i assume it must be better than that. either way you're constantly interrupting and context switching between the target code you're trying to run and the translation environment, context switches are expensive in and of themselves, and they also defeat a lot of caching)

It’s because Rosetta is able to do AOT compilation for the most part. So it actually converts the binary rather than emulating it at run time.

Correct, plus Rosetta is substantially faster than QEMU because of AOT (as others mentioned) as well as a greater focus on performance.

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