Crate orx_concurrent_iter

orx-concurrent-iter

A thread-safe, convenient and lightweight concurrent iterator trait and efficient implementations.

• convenient: An iterator implementing ConcurrentIter can safely be shared among threads as a shared reference. Further, it may be used similar to a regular Iterator with for syntax.
• efficient and lightweight: All concurrent iterator implementations provided in this crate extend atomic iterators, which are lock-free and depend only on atomic primitives.

A. Examples

Basic Usage

A ConcurrentIter can be safely shared among threads and iterated over concurrently. As expected, it will yield each element only once and in order. The yielded elements will be shared among the threads which concurrently iterates based on first come first serve. In other words, threads concurrently pull elements from the iterator.

use orx_concurrent_iter::*;
use std::fmt::Debug;

fn fake_work<T: Debug>(_x: T) {
    std::thread::sleep(std::time::Duration::from_nanos(10));
}

/// `process` elements of `con_iter` concurrently using `num_threads`
fn process_concurrently<T, ConIter, Fun>(
    process: &Fun,
    num_threads: usize,
    concurrent_iter: ConIter,
) where
    T: Send + Sync,
    Fun: Fn(T) + Send + Sync,
    ConIter: ConcurrentIter<Item = T>,
{
    // just take a reference and share among threads
    let con_iter = &concurrent_iter;

    std::thread::scope(|s| {
        for _ in 0..num_threads {
            s.spawn(move || {
                // concurrently iterate over values in a `for` loop
                for value in con_iter.values() {
                    process(value);
                }
            });
        }
    });
}

/// just fix process and num_threads for brevity
fn con_run<T: Send + Sync + Debug>(con_iter: impl ConcurrentIter<Item = T>) {
    process_concurrently(&fake_work, 8, con_iter)
}

// non-consuming iteration over references
let names: [String; 3] = [
    String::from("foo"),
    String::from("bar"),
    String::from("baz"),
];
con_run::<&String>(names.con_iter());

let values: Vec<i32> = (0..8).map(|x| 3 * x + 1).collect();
con_run::<&i32>(values.con_iter());

let slice: &[i32] = values.as_slice();
con_run::<&i32>(slice.con_iter());

// consuming iteration over values
con_run::<String>(names.into_con_iter());
con_run::<i32>(values.into_con_iter());

// any Iterator into ConcurrentIter
let values: Vec<i32> = (0..1024).collect();

let iter_ref = values.iter().filter(|x| *x % 2 == 0);
con_run::<&i32>(iter_ref.into_con_iter());

let iter_val = values
    .iter()
    .filter(|x| *x % 2 == 0)
    .map(|x| (7 * x + 3) as usize)
    .skip(2)
    .take(5);

con_run::<usize>(iter_val.into_con_iter());

ConcurrentIter::next method is the concurrent counterpart of Iterator::next method, which can be called by a shared reference. Note that regular Iterators cannot be consumed by multiple threads due to &mut self requirement, and hence, ConcurrentIter does not implement Iterator. However, it can be used in a similar way as follows:

while let Some(value) = con_iter.next() {
    process(value);
}

The values method returns a regular Iterator which does nothing but wrap the ConcurrentIter and call next. Its only purpose is to enable using the concurrent iterator directly inside a for loop.

for value in rf_con_iter.values() {
    process(value);
}

Simple Parallel Computing

Considering the elements of the iteration as inputs of a process, ConcurrentIter conveniently allows distribution of tasks to multiple threads.

use orx_concurrent_iter::*;

fn compute(input: u64) -> u64 {
    std::thread::sleep(std::time::Duration::from_nanos(2));
    input
}

fn fold(aggregated: u64, value: u64) -> u64 {
    aggregated + value
}

fn parallel_fold(num_threads: usize, inputs_iter: impl ConcurrentIter<Item = u64>) -> u64 {
    let inputs = &inputs_iter;
    let mut global_result = 0u64;

    std::thread::scope(|s| {
        let handles: Vec<_> = (0..num_threads)
            .map(|_| s.spawn(move || inputs.values().map(compute).fold(0u64, fold)))
            .collect();

        for h in handles {
            let thread_result = h.join().expect("o");
            global_result = fold(global_result, thread_result);
        }
    });
    global_result
}

// test
for num_threads in [1, 2, 4, 8] {
    let values = (0..1024).map(|x| 2 * x);
    assert_eq!(
        parallel_fold(num_threads, values.into_con_iter()),
        1023 * 1024
    );
}

Note that parallel map can also be implemented by merging returned transformed collections, such as vectors. Especially for larger data types, a more efficient approach could be to pair ConcurrentIter with a concurrent collection such as orx_concurrent_bag::ConcurrentBag which allows to efficiently collect results concurrently without copies.

use orx_concurrent_iter::*;
use orx_concurrent_bag::*;

type Input = u64;
type Output = [u64; 1];

fn map(input: Input) -> Output {
    [input]
}

fn parallel_map(
    num_threads: usize,
    inputs_iter: impl ConcurrentIter<Item = u64>,
) -> SplitVec<Output> {
    let output_bag = ConcurrentBag::new();
    let outputs = &output_bag;
    let inputs = &inputs_iter;

    std::thread::scope(|s| {
        for _ in 0..num_threads {
            s.spawn(move || {
                for output in inputs.values().map(map) {
                    outputs.push(output);
                }
            });
        }
    });
    output_bag.into_inner()
}

// test
for num_threads in [1, 2, 4, 8] {
    let inputs = (0..1024).map(|x| 2 * x);
    let outputs = parallel_map(num_threads, inputs.into_con_iter());
    assert_eq!(1024, outputs.len())
}

Note that due to parallelization, outputs is not guaranteed to be in the same order as inputs. In order to preserve the order of the input in the output, ConcurrentIter::ids_and_values method can be used, rather than the values, to get indices of values while iterating (see the explanation in the next section).

Iteration with Indices

In a single-threaded regular Iterator, values can be paired up with their indices easily by calling enumerate on the iterator. We can also call rf_inputs.values().enumerate(); however, this would have a different meaning in a multi-threaded execution. It would pair up the values with the indices of the iteration local to that thread. In other words, the first value of every thread will be paired up with index zero.

However, it is often useful to know the input index of the iteration value. This can be achieved simply by using ConcurrentIter::ids_and_values instead of ConcurrentIter::values.

use orx_concurrent_iter::*;

fn get_all_indices(num_threads: usize, inputs_iter: impl ConcurrentIter) -> Vec<usize> {
    let mut all_indices = vec![];
    let inputs = &inputs_iter;

    std::thread::scope(|s| {
        let handles: Vec<_> = (0..num_threads)
            .map(move |_| {
                s.spawn(|| {
                    inputs
                        .ids_and_values()
                        .map(|(i, _value)| i)
                        .collect::<Vec<_>>()
                })
            })
            .collect();

        for handle in handles {
            let indices_for_thread = handle.join().expect("-");
            all_indices.extend(indices_for_thread);
        }
    });

    all_indices.sort();
    all_indices
}

// test
let inputs = ['a', 'b', 'c', 'd', 'e', 'f'];
let indices = get_all_indices(4, inputs.into_con_iter());
assert_eq!(indices, [0, 1, 2, 3, 4, 5]);

Notice that:

• indices_for_thread vectors collected for each thread are internally in ascending order. In other words, each thread receives elements in order.
• However, indices_for_thread vectors have gaps corresponding to indices of elements pulled by other threads.
• No index appears more than once in any of the indices_for_thread vectors.
• And union of these vectors give the indices from 0 to n-1 where n is the number of yielded elements.

Iteration in Chunks

In the default iteration using for together with values and ids_and_values methods, the threads pull elements one by one. Note that these iterators internally call ConcurrentIter::next and ConcurrentIter::next_id_and_value methods, respectively.

Further, it is also possible to iterate in chunks with ConcurrentIter::next_chunk and ConcurrentIter::next_id_and_chunk methods. These methods differ from next and next_id_and_value in the following:

• They receive the chunk_size parameter.
• They return an Iterator which yields the next chunk_size, or fewer if there is not sufficient, consecutive elements.
• next_id_and_chunk method additionally returns the index of the first element that the returned iterator will yield. Note that the index of the remaining elements can be known since the iterator will return consecutive elements.

use orx_concurrent_iter::*;

fn lag(millis: u64) {
    std::thread::sleep(std::time::Duration::from_millis(millis));
}

let inputs = vec!['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];
let characters = &inputs.con_iter();

let [first, second] = std::thread::scope(|s| {
    let first = s.spawn(move || {
        let mut chars: Vec<char> = vec![];
        loop {
            let mut chunk = characters.next_chunk(3).peekable();
            match chunk.peek() {
                None => break,
                Some(_) => chars.extend(chunk.copied()),
            }
            lag(100);
        }
        chars
    });

    let second = s.spawn(move || {
        let mut chars: Vec<char> = vec![];
        loop {
            lag(50);

            let mut chunk = characters.next_chunk(3).peekable();
            match chunk.peek() {
                None => break,
                Some(_) => chars.extend(chunk.copied()),
            }

            lag(100);
        }
        chars
    });

    let first = first.join().expect("-");
    let second = second.join().expect("o");
    [first, second]
});

// Events in chronological order:
// * first pulls 3 consecutive elements [a, b, c]
// * second pulls 3 consecutive elements [d, e, f]
// * first pulls remaining 2 consecutive elements [h, i]

assert_eq!(first, ['a', 'b', 'c', 'g', 'h']);
assert_eq!(second, ['d', 'e', 'f']);

B. Traits and Implementors

As discussed so far, the trait of types which can safely be iterated concurrently by multiple threads is ConcurrentIter.

Further, there are two traits which define types that can provide a ConcurrentIter.

• A ConcurrentIterable type implements the con_iter(&self) method which returns a concurrent iterator without consuming the type itself.
• On the other hand, types implementing IntoConcurrentIter trait has the into_con_iter(self) method which consumes and converts the type into a concurrent iterator. Additionally there exists IterIntoConcurrentIter trait which is functionally identical to IntoConcurrentIter and only implemented by regular iterators, separated only to allow for special implementations for vectors and arrays.

The following table summarizes the implementations of the standard types in this crate.

Type	ConcurrentIterable con_iter element type	IntoConcurrentIter into_con_iter element type
&'a [T]	&'a T	&'a T
Range<Idx>	Idx	Idx
Vec<T>	&T	T
[T; N]	&T	T
Iter: Iterator<Item = T>	-	T

License

This library is licensed under MIT license. See LICENSE for details.

Structs

• AtomicCounter
  An atomic counter, simply a wrapper around AtomicUsize with utility methods useful for atomic iterators.
• ConIterIdsAndValues
  A regular Iterator crated from the ConcurrentIter::ids_and_values method.
• ConIterOfArray
  A concurrent iterator over an array, consuming the array and yielding its elements.
• ConIterOfIter
  A regular Iter: Iterator ascended to the concurrent programs with use of atomics.
• ConIterOfSlice
  A concurrent iterator over a slice yielding references to the elements.
• ConIterOfVec
  A concurrent iterator over a vector, consuming the vector and yielding its elements.
• ConIterValues
  A regular Iterator crated from the ConcurrentIter::values method.

Traits

• AtomicIter
  Trait defining a set of concurrent iterators which internally uses an atomic counter of elements to be yielded.
• AtomicIterWithInitialLen
  An atomic counter based iterator with exactly known initial length.
• ConcurrentIter
  Trait defining a concurrent iterator with next and next_id_and_chunk methods which can safely be called my multiple threads concurrently.
• ConcurrentIterable
  A type that is concurrently iterable; i.e., which can provide a ConcurrentIter with the con_iter method.
• ExactSizeConcurrentIter
  A concurrent iterator that knows its exact length.
• IntoConcurrentIter
  A type that can be consumed and turned into a concurrent iterator with into_con_iter method.
• IterIntoConcurrentIter
  An Iterator type that can be consumed and turned into a concurrent iterator with into_con_iter method.