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task_pool.rs
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task_pool.rs
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use std::{
future::Future,
marker::PhantomData,
mem,
pin::Pin,
sync::Arc,
thread::{self, JoinHandle},
};
use concurrent_queue::ConcurrentQueue;
use futures_lite::{future, pin};
use crate::Task;
/// Used to create a [`TaskPool`]
#[derive(Debug, Default, Clone)]
#[must_use]
pub struct TaskPoolBuilder {
/// If set, we'll set up the thread pool to use at most n threads. Otherwise use
/// the logical core count of the system
num_threads: Option<usize>,
/// If set, we'll use the given stack size rather than the system default
stack_size: Option<usize>,
/// Allows customizing the name of the threads - helpful for debugging. If set, threads will
/// be named <thread_name> (<thread_index>), i.e. "MyThreadPool (2)"
thread_name: Option<String>,
}
impl TaskPoolBuilder {
/// Creates a new [`TaskPoolBuilder`] instance
pub fn new() -> Self {
Self::default()
}
/// Override the number of threads created for the pool. If unset, we default to the number
/// of logical cores of the system
pub fn num_threads(mut self, num_threads: usize) -> Self {
self.num_threads = Some(num_threads);
self
}
/// Override the stack size of the threads created for the pool
pub fn stack_size(mut self, stack_size: usize) -> Self {
self.stack_size = Some(stack_size);
self
}
/// Override the name of the threads created for the pool. If set, threads will
/// be named `<thread_name> (<thread_index>)`, i.e. `MyThreadPool (2)`
pub fn thread_name(mut self, thread_name: String) -> Self {
self.thread_name = Some(thread_name);
self
}
/// Creates a new [`TaskPool`] based on the current options.
pub fn build(self) -> TaskPool {
TaskPool::new_internal(
self.num_threads,
self.stack_size,
self.thread_name.as_deref(),
)
}
}
/// A thread pool for executing tasks. Tasks are futures that are being automatically driven by
/// the pool on threads owned by the pool.
#[derive(Debug)]
pub struct TaskPool {
/// The executor for the pool
///
/// This has to be separate from TaskPoolInner because we have to create an Arc<Executor> to
/// pass into the worker threads, and we must create the worker threads before we can create
/// the Vec<Task<T>> contained within TaskPoolInner
executor: Arc<async_executor::Executor<'static>>,
/// Inner state of the pool
threads: Vec<JoinHandle<()>>,
shutdown_tx: async_channel::Sender<()>,
}
impl TaskPool {
thread_local! {
static LOCAL_EXECUTOR: async_executor::LocalExecutor<'static> = async_executor::LocalExecutor::new();
}
/// Create a `TaskPool` with the default configuration.
pub fn new() -> Self {
TaskPoolBuilder::new().build()
}
fn new_internal(
num_threads: Option<usize>,
stack_size: Option<usize>,
thread_name: Option<&str>,
) -> Self {
let (shutdown_tx, shutdown_rx) = async_channel::unbounded::<()>();
let executor = Arc::new(async_executor::Executor::new());
let num_threads = num_threads.unwrap_or_else(num_cpus::get);
let threads = (0..num_threads)
.map(|i| {
let ex = Arc::clone(&executor);
let shutdown_rx = shutdown_rx.clone();
// miri does not support setting thread names
// TODO: change back when https://github.com/rust-lang/miri/issues/1717 is fixed
#[cfg(not(miri))]
let mut thread_builder = {
let thread_name = if let Some(thread_name) = thread_name {
format!("{} ({})", thread_name, i)
} else {
format!("TaskPool ({})", i)
};
thread::Builder::new().name(thread_name)
};
#[cfg(miri)]
let mut thread_builder = {
let _ = i;
let _ = thread_name;
thread::Builder::new()
};
if let Some(stack_size) = stack_size {
thread_builder = thread_builder.stack_size(stack_size);
}
thread_builder
.spawn(move || {
let shutdown_future = ex.run(shutdown_rx.recv());
// Use unwrap_err because we expect a Closed error
future::block_on(shutdown_future).unwrap_err();
})
.expect("Failed to spawn thread.")
})
.collect();
Self {
executor,
threads,
shutdown_tx,
}
}
/// Return the number of threads owned by the task pool
pub fn thread_num(&self) -> usize {
self.threads.len()
}
/// Allows spawning non-`'static` futures on the thread pool. The function takes a callback,
/// passing a scope object into it. The scope object provided to the callback can be used
/// to spawn tasks. This function will await the completion of all tasks before returning.
///
/// This is similar to `rayon::scope` and `crossbeam::scope`
///
/// # Example
///
/// ```
/// use bevy_tasks::TaskPool;
///
/// let pool = TaskPool::new();
/// let mut x = 0;
/// let results = pool.scope(|s| {
/// s.spawn(async {
/// // you can borrow the spawner inside a task and spawn tasks from within the task
/// s.spawn(async {
/// // borrow x and mutate it.
/// x = 2;
/// // return a value from the task
/// 1
/// });
/// // return some other value from the first task
/// 0
/// });
/// });
///
/// // results are returned in the order the tasks are spawned in.
/// // Note: the ordering may become non-deterministic if you spawn from within tasks.
/// // the ordering is only guaranteed when tasks are spawned directly from the main closure.
/// assert_eq!(&results[..], &[0, 1]);
/// // can access x after scope runs
/// assert_eq!(x, 2);
/// ```
///
/// # Lifetimes
///
/// The [`Scope`] object takes two lifetimes: `'scope` and `'env`.
///
/// The `'scope` lifetime represents the lifetime of the scope. That is the time during
/// which the provided closure and tasks that are spawned into the scope are run.
///
/// The `'env` lifetime represents the lifetime of whatever is borrowed by the scope.
/// Thus this lifetime must outlive `'scope`.
///
/// ```compile_fail
/// use bevy_tasks::TaskPool;
/// fn scope_escapes_closure() {
/// let pool = TaskPool::new();
/// let foo = Box::new(42);
/// pool.scope(|scope| {
/// std::thread::spawn(move || {
/// // UB. This could spawn on the scope after `.scope` returns and the internal Scope is dropped.
/// scope.spawn(async move {
/// assert_eq!(*foo, 42);
/// });
/// });
/// });
/// }
/// ```
///
/// ```compile_fail
/// use bevy_tasks::TaskPool;
/// fn cannot_borrow_from_closure() {
/// let pool = TaskPool::new();
/// pool.scope(|scope| {
/// let x = 1;
/// let y = &x;
/// scope.spawn(async move {
/// assert_eq!(*y, 1);
/// });
/// });
/// }
///
pub fn scope<'env, F, T>(&self, f: F) -> Vec<T>
where
F: for<'scope> FnOnce(&'env Scope<'scope, 'env, T>),
T: Send + 'static,
{
// SAFETY: This safety comment applies to all references transmuted to 'env.
// Any futures spawned with these references need to return before this function completes.
// This is guaranteed because we drive all the futures spawned onto the Scope
// to completion in this function. However, rust has no way of knowing this so we
// transmute the lifetimes to 'env here to appease the compiler as it is unable to validate safety.
let executor: &async_executor::Executor = &*self.executor;
let executor: &'env async_executor::Executor = unsafe { mem::transmute(executor) };
let task_scope_executor = &async_executor::Executor::default();
let task_scope_executor: &'env async_executor::Executor =
unsafe { mem::transmute(task_scope_executor) };
let spawned: ConcurrentQueue<async_executor::Task<T>> = ConcurrentQueue::unbounded();
let spawned_ref: &'env ConcurrentQueue<async_executor::Task<T>> =
unsafe { mem::transmute(&spawned) };
let scope = Scope {
executor,
task_scope_executor,
spawned: spawned_ref,
scope: PhantomData,
env: PhantomData,
};
let scope_ref: &'env Scope<'_, 'env, T> = unsafe { mem::transmute(&scope) };
f(scope_ref);
if spawned.is_empty() {
Vec::new()
} else {
let get_results = async move {
let mut results = Vec::with_capacity(spawned.len());
while let Ok(task) = spawned.pop() {
results.push(task.await);
}
results
};
// Pin the futures on the stack.
pin!(get_results);
// SAFETY: This function blocks until all futures complete, so we do not read/write
// the data from futures outside of the 'scope lifetime. However,
// rust has no way of knowing this so we must convert to 'static
// here to appease the compiler as it is unable to validate safety.
let get_results: Pin<&mut (dyn Future<Output = Vec<T>> + 'static + Send)> = get_results;
let get_results: Pin<&'static mut (dyn Future<Output = Vec<T>> + 'static + Send)> =
unsafe { mem::transmute(get_results) };
// The thread that calls scope() will participate in driving tasks in the pool
// forward until the tasks that are spawned by this scope() call
// complete. (If the caller of scope() happens to be a thread in
// this thread pool, and we only have one thread in the pool, then
// simply calling future::block_on(spawned) would deadlock.)
let mut spawned = task_scope_executor.spawn(get_results);
loop {
if let Some(result) = future::block_on(future::poll_once(&mut spawned)) {
break result;
};
self.executor.try_tick();
task_scope_executor.try_tick();
}
}
}
/// Spawns a static future onto the thread pool. The returned Task is a future. It can also be
/// cancelled and "detached" allowing it to continue running without having to be polled by the
/// end-user.
///
/// If the provided future is non-`Send`, [`TaskPool::spawn_local`] should be used instead.
pub fn spawn<T>(&self, future: impl Future<Output = T> + Send + 'static) -> Task<T>
where
T: Send + 'static,
{
Task::new(self.executor.spawn(future))
}
/// Spawns a static future on the thread-local async executor for the current thread. The task
/// will run entirely on the thread the task was spawned on. The returned Task is a future.
/// It can also be cancelled and "detached" allowing it to continue running without having
/// to be polled by the end-user. Users should generally prefer to use [`TaskPool::spawn`]
/// instead, unless the provided future is not `Send`.
pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> Task<T>
where
T: 'static,
{
Task::new(TaskPool::LOCAL_EXECUTOR.with(|executor| executor.spawn(future)))
}
}
impl Default for TaskPool {
fn default() -> Self {
Self::new()
}
}
impl Drop for TaskPool {
fn drop(&mut self) {
self.shutdown_tx.close();
let panicking = thread::panicking();
for join_handle in self.threads.drain(..) {
let res = join_handle.join();
if !panicking {
res.expect("Task thread panicked while executing.");
}
}
}
}
/// A `TaskPool` scope for running one or more non-`'static` futures.
///
/// For more information, see [`TaskPool::scope`].
#[derive(Debug)]
pub struct Scope<'scope, 'env: 'scope, T> {
executor: &'env async_executor::Executor<'env>,
task_scope_executor: &'env async_executor::Executor<'env>,
spawned: &'env ConcurrentQueue<async_executor::Task<T>>,
// make `Scope` invariant over 'scope and 'env
scope: PhantomData<&'scope mut &'scope ()>,
env: PhantomData<&'env mut &'env ()>,
}
impl<'scope, 'env, T: Send + 'env> Scope<'scope, 'env, T> {
/// Spawns a scoped future onto the thread pool. The scope *must* outlive
/// the provided future. The results of the future will be returned as a part of
/// [`TaskPool::scope`]'s return value.
///
/// For futures that should run on the thread `scope` is called on [`Scope::spawn_on_scope`] should be used
/// instead.
///
/// For more information, see [`TaskPool::scope`].
pub fn spawn<Fut: Future<Output = T> + 'env + Send>(&self, f: Fut) {
let task = self.executor.spawn(f);
// ConcurrentQueue only errors when closed or full, but we never
// close and use an unbouded queue, so it is safe to unwrap
self.spawned.push(task).unwrap();
}
/// Spawns a scoped future onto the thread the scope is run on. The scope *must* outlive
/// the provided future. The results of the future will be returned as a part of
/// [`TaskPool::scope`]'s return value. Users should generally prefer to use
/// [`Scope::spawn`] instead, unless the provided future needs to run on the scope's thread.
///
/// For more information, see [`TaskPool::scope`].
pub fn spawn_on_scope<Fut: Future<Output = T> + 'env + Send>(&self, f: Fut) {
let task = self.task_scope_executor.spawn(f);
// ConcurrentQueue only errors when closed or full, but we never
// close and use an unbouded queue, so it is safe to unwrap
self.spawned.push(task).unwrap();
}
}
#[cfg(test)]
#[allow(clippy::blacklisted_name)]
mod tests {
use super::*;
use std::sync::{
atomic::{AtomicBool, AtomicI32, Ordering},
Barrier,
};
#[test]
fn test_spawn() {
let pool = TaskPool::new();
let foo = Box::new(42);
let foo = &*foo;
let count = Arc::new(AtomicI32::new(0));
let outputs = pool.scope(|scope| {
for _ in 0..100 {
let count_clone = count.clone();
scope.spawn(async move {
if *foo != 42 {
panic!("not 42!?!?")
} else {
count_clone.fetch_add(1, Ordering::Relaxed);
*foo
}
});
}
});
for output in &outputs {
assert_eq!(*output, 42);
}
assert_eq!(outputs.len(), 100);
assert_eq!(count.load(Ordering::Relaxed), 100);
}
#[test]
fn test_mixed_spawn_on_scope_and_spawn() {
let pool = TaskPool::new();
let foo = Box::new(42);
let foo = &*foo;
let local_count = Arc::new(AtomicI32::new(0));
let non_local_count = Arc::new(AtomicI32::new(0));
let outputs = pool.scope(|scope| {
for i in 0..100 {
if i % 2 == 0 {
let count_clone = non_local_count.clone();
scope.spawn(async move {
if *foo != 42 {
panic!("not 42!?!?")
} else {
count_clone.fetch_add(1, Ordering::Relaxed);
*foo
}
});
} else {
let count_clone = local_count.clone();
scope.spawn_on_scope(async move {
if *foo != 42 {
panic!("not 42!?!?")
} else {
count_clone.fetch_add(1, Ordering::Relaxed);
*foo
}
});
}
}
});
for output in &outputs {
assert_eq!(*output, 42);
}
assert_eq!(outputs.len(), 100);
assert_eq!(local_count.load(Ordering::Relaxed), 50);
assert_eq!(non_local_count.load(Ordering::Relaxed), 50);
}
#[test]
fn test_thread_locality() {
let pool = Arc::new(TaskPool::new());
let count = Arc::new(AtomicI32::new(0));
let barrier = Arc::new(Barrier::new(101));
let thread_check_failed = Arc::new(AtomicBool::new(false));
for _ in 0..100 {
let inner_barrier = barrier.clone();
let count_clone = count.clone();
let inner_pool = pool.clone();
let inner_thread_check_failed = thread_check_failed.clone();
std::thread::spawn(move || {
inner_pool.scope(|scope| {
let inner_count_clone = count_clone.clone();
scope.spawn(async move {
inner_count_clone.fetch_add(1, Ordering::Release);
});
let spawner = std::thread::current().id();
let inner_count_clone = count_clone.clone();
scope.spawn_on_scope(async move {
inner_count_clone.fetch_add(1, Ordering::Release);
if std::thread::current().id() != spawner {
// NOTE: This check is using an atomic rather than simply panicing the
// thread to avoid deadlocking the barrier on failure
inner_thread_check_failed.store(true, Ordering::Release);
}
});
});
inner_barrier.wait();
});
}
barrier.wait();
assert!(!thread_check_failed.load(Ordering::Acquire));
assert_eq!(count.load(Ordering::Acquire), 200);
}
#[test]
fn test_nested_spawn() {
let pool = TaskPool::new();
let foo = Box::new(42);
let foo = &*foo;
let count = Arc::new(AtomicI32::new(0));
let outputs: Vec<i32> = pool.scope(|scope| {
for _ in 0..10 {
let count_clone = count.clone();
scope.spawn(async move {
for _ in 0..10 {
let count_clone_clone = count_clone.clone();
scope.spawn(async move {
if *foo != 42 {
panic!("not 42!?!?")
} else {
count_clone_clone.fetch_add(1, Ordering::Relaxed);
*foo
}
});
}
*foo
});
}
});
for output in &outputs {
assert_eq!(*output, 42);
}
// the inner loop runs 100 times and the outer one runs 10. 100 + 10
assert_eq!(outputs.len(), 110);
assert_eq!(count.load(Ordering::Relaxed), 100);
}
#[test]
fn test_nested_locality() {
let pool = Arc::new(TaskPool::new());
let count = Arc::new(AtomicI32::new(0));
let barrier = Arc::new(Barrier::new(101));
let thread_check_failed = Arc::new(AtomicBool::new(false));
for _ in 0..100 {
let inner_barrier = barrier.clone();
let count_clone = count.clone();
let inner_pool = pool.clone();
let inner_thread_check_failed = thread_check_failed.clone();
std::thread::spawn(move || {
inner_pool.scope(|scope| {
let spawner = std::thread::current().id();
let inner_count_clone = count_clone.clone();
scope.spawn(async move {
inner_count_clone.fetch_add(1, Ordering::Release);
// spawning on the scope from another thread runs the futures on the scope's thread
scope.spawn_on_scope(async move {
inner_count_clone.fetch_add(1, Ordering::Release);
if std::thread::current().id() != spawner {
// NOTE: This check is using an atomic rather than simply panicing the
// thread to avoid deadlocking the barrier on failure
inner_thread_check_failed.store(true, Ordering::Release);
}
});
});
});
inner_barrier.wait();
});
}
barrier.wait();
assert!(!thread_check_failed.load(Ordering::Acquire));
assert_eq!(count.load(Ordering::Acquire), 200);
}
}