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diff --git a/src/iter/plumbing/README.md b/src/iter/plumbing/README.md new file mode 100644 index 0000000..cd94eae --- /dev/null +++ b/src/iter/plumbing/README.md @@ -0,0 +1,315 @@ +# Parallel Iterators + +These are some notes on the design of the parallel iterator traits. +This file does not describe how to **use** parallel iterators. + +## The challenge + +Parallel iterators are more complicated than sequential iterators. +The reason is that they have to be able to split themselves up and +operate in parallel across the two halves. + +The current design for parallel iterators has two distinct modes in +which they can be used; as we will see, not all iterators support both +modes (which is why there are two): + +- **Pull mode** (the `Producer` and `UnindexedProducer` traits): in this mode, + the iterator is asked to produce the next item using a call to `next`. This + is basically like a normal iterator, but with a twist: you can split the + iterator in half to produce disjoint items in separate threads. + - in the `Producer` trait, splitting is done with `split_at`, which accepts + an index where the split should be performed. Only indexed iterators can + work in this mode, as they know exactly how much data they will produce, + and how to locate the requested index. + - in the `UnindexedProducer` trait, splitting is done with `split`, which + simply requests that the producer divide itself *approximately* in half. + This is useful when the exact length and/or layout is unknown, as with + `String` characters, or when the length might exceed `usize`, as with + `Range<u64>` on 32-bit platforms. + - In theory, any `Producer` could act unindexed, but we don't currently + use that possibility. When you know the exact length, a `split` can + simply be implemented as `split_at(length/2)`. +- **Push mode** (the `Consumer` and `UnindexedConsumer` traits): in + this mode, the iterator instead is *given* each item in turn, which + is then processed. This is the opposite of a normal iterator. It's + more like a `for_each` call: each time a new item is produced, the + `consume` method is called with that item. (The traits themselves are + a bit more complex, as they support state that can be threaded + through and ultimately reduced.) Unlike producers, there are two + variants of consumers. The difference is how the split is performed: + - in the `Consumer` trait, splitting is done with `split_at`, which + accepts an index where the split should be performed. All + iterators can work in this mode. The resulting halves thus have an + idea about how much data they expect to consume. + - in the `UnindexedConsumer` trait, splitting is done with + `split_off_left`. There is no index: the resulting halves must be + prepared to process any amount of data, and they don't know where that + data falls in the overall stream. + - Not all consumers can operate in this mode. It works for + `for_each` and `reduce`, for example, but it does not work for + `collect_into_vec`, since in that case the position of each item is + important for knowing where it ends up in the target collection. + +## How iterator execution proceeds + +We'll walk through this example iterator chain to start. This chain +demonstrates more-or-less the full complexity of what can happen. + +```rust +vec1.par_iter() + .zip(vec2.par_iter()) + .flat_map(some_function) + .for_each(some_other_function) +``` + +To handle an iterator chain, we start by creating consumers. This +works from the end. So in this case, the call to `for_each` is the +final step, so it will create a `ForEachConsumer` that, given an item, +just calls `some_other_function` with that item. (`ForEachConsumer` is +a very simple consumer because it doesn't need to thread any state +between items at all.) + +Now, the `for_each` call will pass this consumer to the base iterator, +which is the `flat_map`. It will do this by calling the `drive_unindexed` +method on the `ParallelIterator` trait. `drive_unindexed` basically +says "produce items for this iterator and feed them to this consumer"; +it only works for unindexed consumers. + +(As an aside, it is interesting that only some consumers can work in +unindexed mode, but all producers can *drive* an unindexed consumer. +In contrast, only some producers can drive an *indexed* consumer, but +all consumers can be supplied indexes. Isn't variance neat.) + +As it happens, `FlatMap` only works with unindexed consumers anyway. +This is because flat-map basically has no idea how many items it will +produce. If you ask flat-map to produce the 22nd item, it can't do it, +at least not without some intermediate state. It doesn't know whether +processing the first item will create 1 item, 3 items, or 100; +therefore, to produce an arbitrary item, it would basically just have +to start at the beginning and execute sequentially, which is not what +we want. But for unindexed consumers, this doesn't matter, since they +don't need to know how much data they will get. + +Therefore, `FlatMap` can wrap the `ForEachConsumer` with a +`FlatMapConsumer` that feeds to it. This `FlatMapConsumer` will be +given one item. It will then invoke `some_function` to get a parallel +iterator out. It will then ask this new parallel iterator to drive the +`ForEachConsumer`. The `drive_unindexed` method on `flat_map` can then +pass the `FlatMapConsumer` up the chain to the previous item, which is +`zip`. At this point, something interesting happens. + +## Switching from push to pull mode + +If you think about `zip`, it can't really be implemented as a +consumer, at least not without an intermediate thread and some +channels or something (or maybe coroutines). The problem is that it +has to walk two iterators *in lockstep*. Basically, it can't call two +`drive` methods simultaneously, it can only call one at a time. So at +this point, the `zip` iterator needs to switch from *push mode* into +*pull mode*. + +You'll note that `Zip` is only usable if its inputs implement +`IndexedParallelIterator`, meaning that they can produce data starting +at random points in the stream. This need to switch to push mode is +exactly why. If we want to split a zip iterator at position 22, we +need to be able to start zipping items from index 22 right away, +without having to start from index 0. + +Anyway, so at this point, the `drive_unindexed` method for `Zip` stops +creating consumers. Instead, it creates a *producer*, a `ZipProducer`, +to be exact, and calls the `bridge` function in the `internals` +module. Creating a `ZipProducer` will in turn create producers for +the two iterators being zipped. This is possible because they both +implement `IndexedParallelIterator`. + +The `bridge` function will then connect the consumer, which is +handling the `flat_map` and `for_each`, with the producer, which is +handling the `zip` and its preecessors. It will split down until the +chunks seem reasonably small, then pull items from the producer and +feed them to the consumer. + +## The base case + +The other time that `bridge` gets used is when we bottom out in an +indexed producer, such as a slice or range. There is also a +`bridge_unindexed` equivalent for - you guessed it - unindexed producers, +such as string characters. + +<a name="producer-callback"> + +## What on earth is `ProducerCallback`? + +We saw that when you call a parallel action method like +`par_iter.reduce()`, that will create a "reducing" consumer and then +invoke `par_iter.drive_unindexed()` (or `par_iter.drive()`) as +appropriate. This may create yet more consumers as we proceed up the +parallel iterator chain. But at some point we're going to get to the +start of the chain, or to a parallel iterator (like `zip()`) that has +to coordinate multiple inputs. At that point, we need to start +converting parallel iterators into producers. + +The way we do this is by invoking the method `with_producer()`, defined on +`IndexedParallelIterator`. This is a callback scheme. In an ideal world, +it would work like this: + +```rust +base_iter.with_producer(|base_producer| { + // here, `base_producer` is the producer for `base_iter` +}); +``` + +In that case, we could implement a combinator like `map()` by getting +the producer for the base iterator, wrapping it to make our own +`MapProducer`, and then passing that to the callback. Something like +this: + +```rust +struct MapProducer<'f, P, F: 'f> { + base: P, + map_op: &'f F, +} + +impl<I, F> IndexedParallelIterator for Map<I, F> + where I: IndexedParallelIterator, + F: MapOp<I::Item>, +{ + fn with_producer<CB>(self, callback: CB) -> CB::Output { + let map_op = &self.map_op; + self.base_iter.with_producer(|base_producer| { + // Here `producer` is the producer for `self.base_iter`. + // Wrap that to make a `MapProducer` + let map_producer = MapProducer { + base: base_producer, + map_op: map_op + }; + + // invoke the callback with the wrapped version + callback(map_producer) + }); + } +}); +``` + +This example demonstrates some of the power of the callback scheme. +It winds up being a very flexible setup. For one thing, it means we +can take ownership of `par_iter`; we can then in turn give ownership +away of its bits and pieces into the producer (this is very useful if +the iterator owns an `&mut` slice, for example), or create shared +references and put *those* in the producer. In the case of map, for +example, the parallel iterator owns the `map_op`, and we borrow +references to it which we then put into the `MapProducer` (this means +the `MapProducer` can easily split itself and share those references). +The `with_producer` method can also create resources that are needed +during the parallel execution, since the producer does not have to be +returned. + +Unfortunately there is a catch. We can't actually use closures the way +I showed you. To see why, think about the type that `map_producer` +would have to have. If we were going to write the `with_producer` +method using a closure, it would have to look something like this: + +```rust +pub trait IndexedParallelIterator: ParallelIterator { + type Producer; + fn with_producer<CB, R>(self, callback: CB) -> R + where CB: FnOnce(Self::Producer) -> R; + ... +} +``` + +Note that we had to add this associated type `Producer` so that +we could specify the argument of the callback to be `Self::Producer`. +Now, imagine trying to write that `MapProducer` impl using this style: + +```rust +impl<I, F> IndexedParallelIterator for Map<I, F> + where I: IndexedParallelIterator, + F: MapOp<I::Item>, +{ + type MapProducer = MapProducer<'f, P::Producer, F>; + // ^^ wait, what is this `'f`? + + fn with_producer<CB, R>(self, callback: CB) -> R + where CB: FnOnce(Self::Producer) -> R + { + let map_op = &self.map_op; + // ^^^^^^ `'f` is (conceptually) the lifetime of this reference, + // so it will be different for each call to `with_producer`! + } +} +``` + +This may look familiar to you: it's the same problem that we have +trying to define an `Iterable` trait. Basically, the producer type +needs to include a lifetime (here, `'f`) that refers to the body of +`with_producer` and hence is not in scope at the impl level. + +If we had [associated type constructors][1598], we could solve this +problem that way. But there is another solution. We can use a +dedicated callback trait like `ProducerCallback`, instead of `FnOnce`: + +[1598]: https://github.com/rust-lang/rfcs/pull/1598 + +```rust +pub trait ProducerCallback<T> { + type Output; + fn callback<P>(self, producer: P) -> Self::Output + where P: Producer<Item=T>; +} +``` + +Using this trait, the signature of `with_producer()` looks like this: + +```rust +fn with_producer<CB: ProducerCallback<Self::Item>>(self, callback: CB) -> CB::Output; +``` + +Notice that this signature **never has to name the producer type** -- +there is no associated type `Producer` anymore. This is because the +`callback()` method is generically over **all** producers `P`. + +The problem is that now the `||` sugar doesn't work anymore. So we +have to manually create the callback struct, which is a mite tedious. +So our `MapProducer` code looks like this: + +```rust +impl<I, F> IndexedParallelIterator for Map<I, F> + where I: IndexedParallelIterator, + F: MapOp<I::Item>, +{ + fn with_producer<CB>(self, callback: CB) -> CB::Output + where CB: ProducerCallback<Self::Item> + { + return self.base.with_producer(Callback { callback: callback, map_op: self.map_op }); + // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + // Manual version of the closure sugar: create an instance + // of a struct that implements `ProducerCallback`. + + // The struct declaration. Each field is something that need to capture from the + // creating scope. + struct Callback<CB, F> { + callback: CB, + map_op: F, + } + + // Implement the `ProducerCallback` trait. This is pure boilerplate. + impl<T, F, CB> ProducerCallback<T> for Callback<CB, F> + where F: MapOp<T>, + CB: ProducerCallback<F::Output> + { + type Output = CB::Output; + + fn callback<P>(self, base: P) -> CB::Output + where P: Producer<Item=T> + { + // The body of the closure is here: + let producer = MapProducer { base: base, + map_op: &self.map_op }; + self.callback.callback(producer) + } + } + } +} +``` + +OK, a bit tedious, but it works! |