内容简介：As of stableA very basic simple coroutine library contains only an event-less 'yield' primitive, which stops execution of the current coroutine so that other coroutines can run. This is the kind of library that I chose to demonstrate in this post to provid
As of stable Rust
1.39.0, it is possible to
implement a very basic and safe coroutine
library using Rust's
async
/ await
support, and
in under 100 lines of code. The implementation depends solely on std
and is
stack-less (meaning, not depending on an separate CPU architecture stack).
A very basic simple coroutine library contains only an event-less 'yield' primitive, which stops execution of the current coroutine so that other coroutines can run. This is the kind of library that I chose to demonstrate in this post to provide the most concise example.
Yielder
To the coroutine we pass a Fib
struct that only contains a simple binary
state. This Fib
struct has a waiter
method that creates a
Future
that the
coroutine can use in order to be await
ed upon.
use std::future::Future;
use std::pin::Pin;
use std::task::{Poll, Context};
enum State {
Halted,
Running,
}
struct Fib {
state: State,
}
impl Fib {
fn waiter<'a>(&'a mut self) -> Waiter<'a> {
Waiter { fib: self }
}
}
struct Waiter<'a> {
fib: &'a mut Fib,
}
impl<'a> Future for Waiter<'a> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, _cx: &mut Context) -> Poll<Self::Output> {
match self.fib.state {
State::Halted => {
self.fib.state = State::Running;
Poll::Ready(())
}
State::Running => {
self.fib.state = State::Halted;
Poll::Pending
}
}
}
}
Executor
Our executor keeps a vector of uncompleted futures, where the state of each
future is located on the heap. As a very basic executor, it only supports
adding futures to it before actual execution takes place and not afterward.
The push
method adds a closure to the list of futures, and the run
method
performs interlaced execution of futures until all of them complete.
use std::collections::VecDeque;
struct Executor {
fibs: VecDeque<Pin<Box<dyn Future<Output=()>>>>,
}
impl Executor {
fn new() -> Self {
Executor {
fibs: VecDeque::new(),
}
}
fn push<C, F>(&mut self, closure: C)
where
F: Future<Output=()> + 'static,
C: FnOnce(Fib) -> F,
{
let fib = Fib { state: State::Running };
self.fibs.push_back(Box::pin(closure(fib)));
}
fn run(&mut self) {
let waker = waker::create();
let mut context = Context::from_waker(&waker);
while let Some(mut fib) = self.fibs.pop_front() {
match fib.as_mut().poll(&mut context) {
Poll::Pending => {
self.fibs.push_back(fib);
},
Poll::Ready(()) => {},
}
}
}
}
Null Waker
For the executor implementation above, we need a null Waker , similar to the one used in genawaiter ( link ).
use std::task::{RawWaker, RawWakerVTable, Waker},
pub fn create() -> Waker {
// Safety: The waker points to a vtable with functions that do nothing. Doing
// nothing is memory-safe.
unsafe { Waker::from_raw(RAW_WAKER) }
}
const RAW_WAKER: RawWaker = RawWaker::new(std::ptr::null(), &VTABLE);
const VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake_by_ref, drop);
unsafe fn clone(_: *const ()) -> RawWaker { RAW_WAKER }
unsafe fn wake(_: *const ()) { }
unsafe fn wake_by_ref(_: *const ()) { }
unsafe fn drop(_: *const ()) { }
Giving it a go
We can test the library using a program such as the following:
pub fn main() {
let mut exec = Executor::new();
for instance in 1..=3 {
exec.push(move |mut fib| async move {
println!("{} A", instance);
fib.waiter().await;
println!("{} B", instance);
fib.waiter().await;
println!("{} C", instance);
fib.waiter().await;
println!("{} D", instance);
});
}
println!("Running");
exec.run();
println!("Done");
}
The output is as follows:
Running 1 A 2 A 3 A 1 B 2 B 3 B 1 C 2 C 3 C 1 D 2 D 3 D Done
Performance
Timing the following program compiled with lto = true
, I have seen that it
takes about 5 nanoseconds for each iteration of the internal loop, on an Intel
i7-7820X CPU.
pub fn bench() {
let mut exec = Executor::new();
for _ in 1..=2 {
exec.push(move |mut fib| async move {
for _ in 0..100_000_000 {
fib.waiter().await;
}
});
}
println!("Running");
exec.run();
println!("Done");
}
End notes
One of the nice things about the async
/ await
support in the Rust compiler
is that it does not depend on any specific run-time. Thus, if you commit to
certain run-time, you are free to implement your own executor.
Independency of run-time has its downsides. For example, the library presented
here is not compatible with other run-times such as
async-std
. And in fact, the implementation
violates the interface intended for the Future
's poll
function by assuming
that the Future
will always be Ready
after it was Pending
.
Combined uses of several run-times in a single program is possible but requires extra care (see Reddit discussion ).
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