tokio/tokio-threadpool/src/lib.rs

#![doc(html_root_url = "https://docs.rs/tokio-threadpool/0.1.5")]
#![deny(warnings, missing_docs, missing_debug_implementations)]

//! A work-stealing based thread pool for executing futures.
//!
//! The Tokio thread pool supports scheduling futures and processing them on
//! multiple CPU cores. It is optimized for the primary Tokio use case of many
//! independent tasks with limited computation and with most tasks waiting on
//! I/O. Usually, users will not create a `ThreadPool` instance directly, but
//! will use one via a [`runtime`].
//!
//! The `TheadPool` structure manages two sets of threads:
//!
//! * Worker threads.
//! * Backup threads.
//!
//! Worker threads are used to schedule futures using a work-stealing strategy.
//! Backup threads, on the other hand, are intended only to support the
//! `blocking` API. Threads will transition between the two sets.
//!
//! The advantage of the work-stealing strategy is minimal cross-thread
//! coordination. The thread pool attempts to make as much progress as possible
//! without communicating across threads.
//!
//! ## Worker overview
//!
//! Each worker has two queues: a deque and a mpsc channel. The deque is the
//! primary queue for tasks that are scheduled to run on the worker thread. Tasks
//! can only be pushed onto the deque by the worker, but other workers may
//! "steal" from that deque. The mpsc channel is used to submit futures while
//! external to the pool.
//!
//! As long as the thread pool has not been shutdown, a worker will run in a
//! loop. Each loop, it consumes all tasks on its mpsc channel and pushes it onto
//! the deque. It then pops tasks off of the deque and executes them.
//!
//! If a worker has no work, i.e., both queues are empty. It attempts to steal.
//! To do this, it randomly scans other workers' deques and tries to pop a task.
//! If it finds no work to steal, the thread goes to sleep.
//!
//! When the worker detects that the pool has been shut down, it exits the loop,
//! cleans up its state, and shuts the thread down.
//!
//! ## Thread pool initialization
//!
//! Note, users normally will use the threadpool created by a [`runtime`].
//!
//! By default, no threads are spawned on creation. Instead, when new futures are
//! spawned, the pool first checks if there are enough active worker threads. If
//! not, a new worker thread is spawned.
//!
//! ## Spawning futures
//!
//! The spawning behavior depends on whether a future was spawned from within a
//! worker or thread or if it was spawned from an external handle.
//!
//! When spawning a future while external to the thread pool, the current
//! strategy is to randomly pick a worker to submit the task to. The task is then
//! pushed onto that worker's mpsc channel.
//!
//! When spawning a future while on a worker thread, the task is pushed onto the
//! back of the current worker's deque.
//!
//! ## Blocking annotation strategy
//!
//! The [`blocking`] function is used to annotate a section of code that
//! performs a blocking operation, either by issuing a blocking syscall or
//! performing any long running CPU-bound computation.
//!
//! The strategy for handling blocking closures is to hand off the worker to a
//! new thread. This implies handing off the `deque` and `mpsc`. Once this is
//! done, the new thread continues to process the work queue and the original
//! thread is able to block. Once it finishes processing the blocking future, the
//! thread has no additional work and is inserted into the backup pool. This
//! makes it available to other workers that encounter a [`blocking`] call.
//!
//! [`blocking`]: fn.blocking.html
//! [`runtime`]: https://docs.rs/tokio/0.1/tokio/runtime/

extern crate tokio_executor;

extern crate crossbeam_deque as deque;
extern crate crossbeam_utils;
#[macro_use]
extern crate futures;
extern crate num_cpus;
extern crate rand;

#[macro_use]
extern crate log;

// ## Crate layout
//
// The primary type, `Pool`, holds the majority of a thread pool's state,
// including the state for each worker. Each worker's state is maintained in an
// instance of `worker::Entry`.
//
// `Worker` contains the logic that runs on each worker thread. It holds an
// `Arc` to `Pool` and is able to access its state from `Pool`.
//
// `Task` is a harness around an individual future. It manages polling and
// scheduling that future.
//
// ## Sleeping workers
//
// Sleeping workers are tracked using a [treiber stack]. This results in the
// thread that most recently went to sleep getting woken up first. When the pool
// is not under load, this helps threads shutdown faster.
//
// Sleeping is done by using `tokio_executor::Park` implementations. This allows
// the user of the thread pool to customize the work that is performed to sleep.
// This is how injecting timers and other functionality into the thread pool is
// done.
//
// ## Notifying workers
//
// When there is work to be done, workers must be notified. However, notifying a
// worker requires cross thread coordination. Ideally, a worker would only be
// notified when it is sleeping, but there is no way to know if a worker is
// sleeping without cross thread communication.
//
// The two cases when a worker might need to be notified are:
//
// 1. A task is externally submitted to a worker via the mpsc channel.
// 2. A worker has a back log of work and needs other workers to steal from it.
//
// In the first case, the worker will always be notified. However, it could be
// possible to avoid the notification if the mpsc channel has two or greater
// number of tasks *after* the task is submitted. In this case, we are able to
// assume that the worker has previously been notified.
//
// The second case is trickier. Currently, whenever a worker spawns a new future
// (pushing it onto its deque) and when it pops a future from its mpsc, it tries
// to notify a sleeping worker to wake up and start stealing. This is a lot of
// notification and it **might** be possible to reduce it.
//
// Also, whenever a worker is woken up via a signal and it does find work, it,
// in turn, will try to wake up a new worker.
//
// [treiber stack]: https://en.wikipedia.org/wiki/Treiber_Stack

pub mod park;

mod blocking;
mod builder;
mod callback;
mod config;
mod notifier;
mod pool;
mod sender;
mod shutdown;
mod shutdown_task;
mod task;
mod thread_pool;
mod worker;

pub use blocking::{blocking, BlockingError};
pub use builder::Builder;
pub use sender::Sender;
pub use shutdown::Shutdown;
pub use thread_pool::ThreadPool;
pub use worker::{Worker, WorkerId};