Re:Async-await on stable Rust
by pron on Friday November 08, 2019 @10:26AM
(#21483026)
You can make what are virtually zero-cost copies from what you call a "big stack" to a resizable stack with virtual memory tricks. You don't even need to copy the entire stack, but cleverly rewrite the return address stored on the stack to do this kind of "code-switching". But it does mean doing backend manipulations in a platform-dependent way. There are several good ways to do this, none of them particularly easy. What is perhaps impossible is allowing FFI code to block the lightweight thread, but async/await doesn't solve this, either.
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> You can make what are virtually zero-cost copies from what you call a "big stack" to a resizable stack with virtual memory tricks. You don't even need to copy the entire stack, but cleverly rewrite the return address stored on the stack to do this kind of "code-switching".
We tried it. It was too slow.
OK. It really is hard when you're what you call "FFI-heavy" and don't like a significant runtime. So Rust has several * self-imposed* constraints (whether they're all essential for its target domains is a separate discussion, but some of those constraints certainly are) that makes this task particularly hard, but my point is that there is nothing fundamental to n:m threading that makes it slower than async/await, and async/await does fundamentally come at the significant cost of a particularly viral form of accidental complexity.