parallel_iterator/lib.rs
1//! parallel-iterator
2//! =================
3//!
4//! A minimal example
5//! -----------------
6//!
7//! This code is copy-pasted from `examples/example_1.rs`.
8//!
9//! ```rust
10//! extern crate parallel_iterator;
11//!
12//! use parallel_iterator::ParallelIterator;
13//!
14//! fn do_some_work(i: u32) -> u32 {
15//! i + 1 // let's pretend this is a heavy calculation
16//! }
17//!
18//! fn main() {
19//! for i in ParallelIterator::new(|| (0u32..100), || do_some_work) {
20//! println!("Got a result: {}!", i);
21//! }
22//! }
23//! ```
24//!
25//!
26//! A _slightly_ more realistic example
27//! -----------------------------------
28//!
29//! This code is copy-pasted from `examples/example_2.rs`.
30//!
31//! ```rust
32//! extern crate parallel_iterator;
33//!
34//! use parallel_iterator::ParallelIterator;
35//!
36//! fn do_some_work(i: usize, out: &mut Vec<usize>) {
37//! for j in 0..i {
38//! out.push(j); // The caller can pre-allocate.
39//! }
40//! }
41//!
42//! fn main() {
43//! const MAX: usize = 1000;
44//! let xform_ctor = || {
45//! let mut buffer = Vec::with_capacity(MAX);
46//! move |i| {
47//! buffer.clear(); // Clear but keep the internal allocation.
48//! do_some_work(i, &mut buffer);
49//! buffer.last().map(|u| *u) // This is just an example value.
50//! }
51//! };
52//! for i in ParallelIterator::new(|| (0..MAX), xform_ctor) {
53//! match i {
54//! Some(i) => println!("Got Some({})!", i),
55//! None => println!("Got None!"),
56//! }
57//! }
58//! }
59//! ```
60//!
61//! Please see the documentation on the ParallelIterator struct for more details.
62
63#![forbid(warnings)]
64#![forbid(unsafe_code)]
65
66extern crate crossbeam_channel;
67extern crate num_cpus;
68
69use crossbeam_channel::bounded;
70use std::marker::Send;
71use std::marker::Sync;
72use std::sync::Arc;
73use std::thread;
74use std::thread::JoinHandle;
75
76pub struct ParallelIterator<O> {
77 channel: crossbeam_channel::IntoIter<O>,
78 threads: Vec<JoinHandle<()>>,
79}
80
81impl<O> ParallelIterator<O> {
82 /// * `PC` - Producer Constructor. Enables usage of !Send and !Sync objects
83 /// in the producer function.
84 ///
85 /// * `XC` - Xform (closure) Constructor. Enables usage of !Send and !Sync
86 /// objects in the transform closure. This can be used for creating thread
87 /// local caches, like large allocations re-used by different tasks,
88 /// packaged as a closure.
89 ///
90 /// * `P` - Producer iterator. Consumed internally by the transform/worker
91 /// threads.
92 ///
93 /// * `X` - Xform closure. Applied to each job item produced by the producer
94 /// iterator, in parallel by multiple worker threads. This can be `FnMut`
95 /// since it's owned by a dedicated worker thread and will never be called
96 /// by some other thread. The closure can safely store and reuse mutable
97 /// resources between job items, for example large memory allocations.
98 ///
99 /// * `I` - Input item. Or task, produced by the producer iterator,
100 /// transformed by the Xform closures.
101 ///
102 /// * `O` - Output item. Returned by the Xform closure(s) and by the
103 /// Iterator::next method.
104 ///
105 pub fn new<PC, XC, P, X, I>(producer_ctor: PC, xform_ctor: XC) -> Self
106 where
107 PC: 'static + Send + FnOnce() -> P,
108 XC: 'static + Send + Sync + Fn() -> X,
109 X: FnMut(I) -> O,
110 I: 'static + Send,
111 O: 'static + Send,
112 P: IntoIterator<Item = I>,
113 {
114 let mut threads = vec![];
115 let jobs_rx = {
116 let (tx, rx) = bounded(1);
117 let join_handle = thread::spawn(move || {
118 for e in producer_ctor() {
119 // Using expect here since this is most likely a fatal error
120 // and the panic should propagate to parent thread.
121 tx.send(e).expect("Producer thread failed to send job.");
122 }
123 });
124 threads.push(join_handle);
125 rx
126 };
127 let results_rx = {
128 let (tx, rx) = bounded(1);
129 let xform_ctor = Arc::new(xform_ctor);
130 for _ in 0..num_cpus::get() {
131 let tx = tx.clone();
132 let jobs_rx = jobs_rx.clone();
133 let xform_ctor = xform_ctor.clone();
134 let join_handle = thread::spawn(move || {
135 let mut xform = xform_ctor();
136 for e in jobs_rx {
137 // Using expect here since this is most likely a fatal
138 // error and the panic should propagate to the parent
139 // thread.
140 tx.send(xform(e))
141 .expect("Worker thread failed to send result.");
142 }
143 });
144 threads.push(join_handle);
145 }
146 rx
147 };
148 Self {
149 channel: results_rx.into_iter(),
150 threads,
151 }
152 }
153
154 fn join_threads(&mut self) {
155 while let Some(join_handle) = self.threads.pop() {
156 // Using expect() here since trying to get the inner panic message
157 // in a typesafe way is not possible?
158 join_handle.join().expect("A child thread has paniced.");
159 }
160 }
161}
162
163impl<T> Iterator for ParallelIterator<T> {
164 type Item = T;
165 fn next(&mut self) -> Option<T> {
166 let item = self.channel.next();
167 if item.is_some() {
168 return item;
169 }
170 self.join_threads();
171 item // Should always be None here.
172 }
173}
174
175#[cfg(test)]
176mod tests {
177 use super::ParallelIterator;
178
179 /// Test helper
180 fn do_some_work(i: u32) -> u32 {
181 (0..1000).fold(i, |acc, x| acc.wrapping_add(x))
182 }
183
184 #[test]
185 fn test_parallel_vs_sequential() {
186 let prod_ctor = || (0u32..100);
187 let xform_ctor = || do_some_work;
188 let result_xform = |acc: u32, x| acc.wrapping_add(x);
189 let prod = prod_ctor();
190 let par_r =
191 ParallelIterator::new(prod_ctor, xform_ctor).fold(0, &result_xform);
192 let seq_r = prod.map(do_some_work).fold(0, &result_xform);
193 assert_eq!(par_r, seq_r);
194 }
195}