> Green threads give you all the advantages of async
They require more memory over stackless coroutines as it stores the callstack instead of changing a single state. They also allow for recursion, but its undelimited meaning you either 1) overrun the guard page and potentially write to another Green thread's stack by just declaring a large local variable 2) enable some form of stack-probing to address that (?) or 3) Support growable stacks which requires a GC to fixup pointes (isn't available in a systems lang).
> green threads should run faster, as storing state on a stack is generally faster than malloc.
Stackless coroutines explicit don't malloc on each call. You only allocate the intial state machine (stack in GreenThread terms).
> The primary objection seems to be speed
It's compatibility. No way to properly set the stack-size at compile time for various platforms. No way to setup guard pages in a construct that's language-level so should support being used without an OS (i.e. embedded, wasm, kernel). The current async using stackless coroutines 1) knows the size upfront due to being a compiler-generated StateMachine 2) disallows recursion (as that's a recursive StateMachine type, so users must dynamically allocate those however appropriate) which works for all targets.
They require more memory over stackless coroutines as it stores the callstack instead of changing a single state. They also allow for recursion, but its undelimited meaning you either 1) overrun the guard page and potentially write to another Green thread's stack by just declaring a large local variable 2) enable some form of stack-probing to address that (?) or 3) Support growable stacks which requires a GC to fixup pointes (isn't available in a systems lang).
> green threads should run faster, as storing state on a stack is generally faster than malloc.
Stackless coroutines explicit don't malloc on each call. You only allocate the intial state machine (stack in GreenThread terms).
> The primary objection seems to be speed
It's compatibility. No way to properly set the stack-size at compile time for various platforms. No way to setup guard pages in a construct that's language-level so should support being used without an OS (i.e. embedded, wasm, kernel). The current async using stackless coroutines 1) knows the size upfront due to being a compiler-generated StateMachine 2) disallows recursion (as that's a recursive StateMachine type, so users must dynamically allocate those however appropriate) which works for all targets.