[pcwalton]

Go is more efficient at M:N then Rust can be mostly for two reasons:

1. Go can start stacks small and relocate them, because the runtime needed to implement garbage collection allows relocation of pointers into the stack by rewriting them. Rust has no such runtime infrastructure: it cannot tell what stack values correspond to pointers and which correspond to other values. Additionally, Rust allows for pointers from the heap into the stack in certain circumstances, which Go does not (I don't think, anyway). So what Rust must do is to reserve a relatively large amount of address space for each thread's stack, because those stacks cannot move in memory. (Note that in the case of Rust the kernel does not have to, and typically does not, actually allocate that much physical memory for each thread until actually needed; the reservation is virtual address space only.) In contrast, Go can start thread stacks small, which makes them faster to allocate, and copy them around as they grow bigger. Note that async/await in Rust has the potential to be more efficient than even Go's stack growth, as the runtime can allocate all the needed space up front in some cases and avoid the copies; this is the consequence of async/await compiling to a static state machine instead of the dynamic control flow that threads have.

2. Rust cares more about fast interoperability with C code that may not be compiled with knowledge of async I/O and stack growth. Go chooses fast M:N threading over this, sacrificing fast FFI in the process as every cgo call needs to switch to a big stack. This is just a tradeoff. Given that 1:1 threading is quite fast and scalable on Linux, it's the right tradeoff for Rust's domain, as losing M:N threading isn't losing that much anyway.