A few months ago I have written a small task system, it looks like this.

let res = TaskPool::submit(|| {
    println!("Before long running task");
    let r = TaskPool::submit(|| {
        std::thread::sleep(Duration::from_secs(10));
        return 42;
    });
    // Waits for the long running task to complete, does not block other tasks!
    println!("After long running task {}", r.await());
    42
});
println!("{}", res.await());
  • It is possible to spawn tasks inside tasks
  • Calling .await() on the main thread will block
  • Calling .await() inside the task pool will reschedule the task

The task system was inspired by Naughty Dog’s Task system but since then future-rs was released. Today I finally had time to test it out.

Instead of spawning tasks inside tasks you create futures.

let some_future = futures::finished::<i32, ()>(42).map(|i| i + 42);

You can think of them as a finite state machine.

let pool = CpuPool::new(3);
let some_future = futures::finished::<i32, ()>(42).map(|i| i + 42);
let cpu_future = pool.spawn(some_future);
println!("{}", cpu_future.wait().unwrap());

The biggest difference here is that submitting and creating work is completely separated.

Futures can be composed together

fn future_test(id: i32) -> impl futures::Future<Item=i32, Error=()> {
    futures::finished::<i32, ()>(42)
        .map(move |i| {
            println!("1st map id {}  {:?}", id, thread_id::get());
            i + 1
        })
        .map(move |i| {
            println!("2nd map id {}  {:?}", id, thread_id::get());
            i + 2
        })
        .map(move |i| {
            println!("3rd map id {}  {:?}", id, thread_id::get());
            i + 3
        })
}

Though you probably want to make use of impl trait if you compose multiple futures together. Above I used the thread-id crate to see on which thread the future will execute.

I am currently writing a rendering engine in Vulkan and I need to record CommandBuffers on different threads. This means I have to figure out how I actually submit futures onto different threads.

let c: Vec<_> = (0 .. 10).map(|i| future_test(i)).collect();
let r = pool.spawn(futures::collect(c));
println!("{:?}", r.wait());

This will print:

1st map id 0  140444805625600
2nd map id 0  140444805625600
3rd map id 0  140444805625600
1st map id 1  140444805625600
2nd map id 1  140444805625600
3rd map id 1  140444805625600
1st map id 2  140444805625600
2nd map id 2  140444805625600
3rd map id 2  140444805625600
1st map id 3  140444805625600
2nd map id 3  140444805625600
3rd map id 3  140444805625600
1st map id 4  140444805625600
2nd map id 4  140444805625600
3rd map id 4  140444805625600
1st map id 5  140444805625600
2nd map id 5  140444805625600
3rd map id 5  140444805625600
1st map id 6  140444805625600
2nd map id 6  140444805625600
3rd map id 6  140444805625600
1st map id 7  140444805625600
2nd map id 7  140444805625600
3rd map id 7  140444805625600
1st map id 8  140444805625600
2nd map id 8  140444805625600
3rd map id 8  140444805625600
1st map id 9  140444805625600
2nd map id 9  140444805625600
3rd map id 9  140444805625600
Ok([48, 48, 48, 48, 48, 48, 48, 48, 48, 48])

You may notice that the thread id is always the same. This is because a future will currently execute only on 1 thread. This is not what I wanted to achieve.

If you want parallelism you should probably not submit one giant future but more smaller ones.

let c: Vec<i32> =
    (0..10).map(|i| pool.spawn(future_test(i))).collect::<Vec<_>>().into_iter()
           .map(|f| f.wait().unwrap()).collect();

This code will print:

1st map id 0  139959187011328
2nd map id 0  139959187011328
3rd map id 0  139959187011328
1st map id 1  139959184910080
2nd map id 1  139959184910080
3rd map id 1  139959184910080
1st map id 2  139959182808832
2nd map id 2  139959182808832
3rd map id 2  139959182808832
1st map id 3  139959187011328
2nd map id 3  139959187011328
3rd map id 3  139959187011328
1st map id 4  139959184910080
2nd map id 4  139959184910080
3rd map id 4  139959184910080
1st map id 5  139959182808832
2nd map id 5  139959182808832
3rd map id 5  139959182808832
1st map id 6  139959187011328
2nd map id 6  139959187011328
3rd map id 6  139959187011328
1st map id 7  139959184910080
2nd map id 7  139959184910080
3rd map id 7  139959184910080
1st map id 8  139959182808832
2nd map id 8  139959182808832
3rd map id 8  139959182808832
1st map id 9  139959187011328
2nd map id 9  139959187011328
3rd map id 9  139959187011328

The code creates 10 Futures from future_test and immediately spawns them with pool.spawn(future_test(i)) which returns a CpuFuture. It then waits sequentially on the result and writes its result into a vector.

You might also notice that every Future from future_test will execute on the same thread.

I haven’t spent too much time with Future-rs but it looks very promising. The next thing I will look into is how I can safely share stack references inside Futures. This was one part where I struggled with my TaskPool implementation and I will probably run into the same issues with Future-rs and TaskPool because spawn has 'static lifetime requirements.

fn spawn<F>(&self, f: F) -> CpuFuture<F::Item, F::Error>
where F: Future + Send + 'static, F::Item: Send + 'static, F::Error: Send + 'static

Rayon does seem to offer this

/// Increment all values in slice.
fn increment_all(slice: &mut [i32]) {
    if slice.len() < 1000 {
        for p in slice { *p += 1; }
    } else {
        let mid_point = slice.len() / 2;
        let (left, right) = slice.split_at_mut(mid_point);
        rayon::join(|| increment_all(left), || increment_all(right));
    }
}

Luckily futures-cpupool is only a few 100 lines and therefore easy to make changes. I might have to remove the Send requirement from .spawn in order to have stack borrows. Mostly because it would be very bad if you create a future that has a borrow on the stack and you send it to a different thread. But it should be safe if it only gets send to the taskpool because you will get back another future which also doesn’t implement Send. Then you could call .wait() in the destructor but I see I am already getting ahead of myself.