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//! Straight-forward functions and types for basic data parallel //! operations. //! //! This library provides a few building blocks for operating on data //! in parallel, particularly iterators. At the moment, it is not //! designed to be robust or eke out every last drop of performance, //! but rather explore some ways in which Rust's type system allows //! for some fairly fancy things to be written with a guarantee of //! safety, all without a garbage collector. //! //! The core design is to simply allow for operations that could occur //! on a single thread to execute on many, it is not intending to //! serve as a hard boundary between threads; in particular, if //! something (a `panic!`) would take down the main thread when run //! sequentially, it will also take down the main thread (eventually) //! when run using the functions in this library. //! //! On the point of performance and robustness, the top level //! functions do no thread pooling and so everything essentially //! spawns a new thread for each element, which is definitely //! suboptimal for many reasons. Fortunately, not all is lost, the //! functionality is designed to be as generic as possible, so the //! iterator functions work with many many iterators, e.g. instead of //! executing a thread on every element of a vector individually, a //! user can divide that vector into disjoint sections and spread //! those across much fewer threads (e.g. [the `chunks` //! method](http://doc.rust-lang.org/nightly/std/slice/trait.SliceExt.html#tymethod.chunks)). //! //! Further, the thread pooling that does exist has a lot of //! synchronisation overhead, and so is actually rarely a performance //! improvement (although it is a robustness improvement over the //! top-level functions, since it limits the number of threads that //! will be spawned). //! //! Either way, **this is not recommended for general use**. //! //! # Usage //! //! This is [available on //! crates.io](https://crates.io/crates/simple_parallel). Add this to //! your Cargo.toml: //! //! ```toml //! [dependencies] //! simple_parallel = "0.3" //! ``` //! //! The latest development version can be obtained [on //! GitHub](https://github.com/huonw/simple_parallel). //! //! # Examples //! //! Initialise an array, in parallel. //! //! ```rust //! # fn foo() { //! let mut data = [0; 10]; //! // fill the array, with one thread for each element: //! simple_parallel::for_(data.iter_mut().enumerate(), |(i, elem)| { //! *elem = i as i32; //! }); //! # } //! //! # let mut data = [0; 10]; //! // now adjust that data, with a threadpool: //! let mut pool = simple_parallel::Pool::new(4); //! pool.for_(data.iter_mut(), |elem| *elem *= 2); //! ``` //! //! Transform each element of an ordered map in a fancy way, in //! parallel, with `map` (`map` ensures the output order matches the //! input order, unlike `unordered_map`), //! //! ```rust //! extern crate crossbeam; //! extern crate simple_parallel; //! //! use std::collections::BTreeMap; //! //! let mut map = BTreeMap::new(); //! map.insert('a', 1); //! map.insert('x', 55); //! //! crossbeam::scope(|scope| { //! // (`IntoIterator` is used, so "direct" iteration like this is fine.) //! let par_iter = simple_parallel::map(scope, &map, |(&c, &elem)| { //! let mut x = elem * c as i32; //! // ... something complicated and expensive ... //! return x as f64 //! }); //! //! // the computation is executing on several threads in the //! // background, so that elements are hopefully ready as soon as //! // possible. //! //! for value in par_iter { //! println!("I computed {}", value); //! } //! }); //! ``` //! //! Sum an arbitrarily long slice, in parallel, by summing subsections and adding //! everything to a shared mutex, stored on the stack of the main //! thread. (A parallel fold is currently missing, hence the mutex.) //! //! ```rust //! use std::sync::Mutex; //! //! // limit the spew of thread spawning to something sensible //! const NUM_CHUNKS: usize = 8; //! //! fn sum(x: &[f64]) -> f64 { //! // (round up) //! let elements_per_chunk = (x.len() + NUM_CHUNKS - 1) / NUM_CHUNKS; //! //! let total = Mutex::new(0.0); //! simple_parallel::for_(x.chunks(elements_per_chunk), |chunk| { //! // sum up this little subsection //! let subsum = chunk.iter().fold(0.0, |a, b| a + *b); //! *total.lock().unwrap() += subsum; //! }); //! //! let answer = *total.lock().unwrap(); //! answer //! } //! ``` //! //! Alternatively, one could use a thread pool, and assign an absolute //! number of elements to each subsection and let the pool manage //! distributing the work among threads, instead of being forced to //! computing the length of the subsections to limit the number of //! threads spawned. //! //! ```rust //! use std::sync::Mutex; //! //! // limit the spew of thread spawning to something sensible //! const ELEMS_PER_JOB: usize = 1_000; //! //! fn pooled_sum(pool: &mut simple_parallel::Pool, x: &[f64]) -> f64 { //! let total = Mutex::new(0.0); //! pool.for_(x.chunks(ELEMS_PER_JOB), |chunk| { //! // sum up this little subsection //! let subsum = chunk.iter().fold(0.0, |a, b| a + *b); //! *total.lock().unwrap() += subsum; //! }); //! //! let answer = *total.lock().unwrap(); //! answer //! } //! ``` //! //! A sketch of a very simple recursive parallel merge-sort, using //! `both` to handle the recursion. (A working implementation may //! really need some temporary buffers to mangle the data, but the key //! point is `both` naturally running things in parallel.) //! //! ```rust //! /// Merges the two sorted runs `left` and `right`. //! /// That is, after `merge(left, right)`, //! /// //! /// left[0] <= left[1] <= ... <= left[last] <= right[0] <= ... //! fn merge<T: Ord>(left: &mut [T], right: &mut [T]) { //! // magic (but non-parallel, so boring) //! } //! //! fn parallel_merge_sort<T: Ord + Send>(x: &mut [T]) { //! // base case //! if x.len() <= 1 { return } //! //! // get two disjoint halves of the `x`, //! let half = x.len() / 2; //! let (left, right) = x.split_at_mut(half); //! // and sort them recursively, in parallel //! simple_parallel::both(&mut *left, &mut *right, |v| parallel_merge_sort(v)); //! //! // now combine the two sorted halves //! merge(left, right) //! } //! ``` //! //! The [`examples` //! folder](https://github.com/huonw/simple_parallel/tree/master/examples) //! contains more intricate example(s), such as a parallel fast //! Fourier transform implementation (it really works, and the //! parallelism does buy something... when tuned). extern crate crossbeam; mod maps; mod fnbox; pub mod pool; pub mod one_to_one { pub use maps::{unordered_map, UnorderedParMap, map, ParMap}; } pub use one_to_one::{map, unordered_map}; pub use pool::Pool; /// Execute `f` on each element of `iter`, in their own `scoped` /// thread. /// /// If `f` panics, so does `for_`. If this occurs, the number of /// elements of `iter` that have had `f` called on them is /// unspecified. pub fn for_<I: IntoIterator, F>(iter: I, ref f: F) where I::Item: Send, F: Fn(I::Item) + Sync { crossbeam::scope(|scope| { for elem in iter { scope.spawn(move || f(elem)); } }); } /// Execute `f` on both `x` and `y`, in parallel, returning the /// result. /// /// This is the same (including panic semantics) as `(f(x), f(y))`, up /// to ordering. It is designed to be used for divide-and-conquer /// algorithms. pub fn both<T, U, F>(x: T, y: T, ref f: F) -> (U, U) where T: Send, U: Send, F: Sync + Fn(T) -> U { crossbeam::scope(|scope| { let guard = scope.spawn(move || f(y)); let a = f(x); let b = guard.join(); (a, b) }) }