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@@ -0,0 +1,227 @@ +# Rayon FAQ + +This file is for general questions that don't fit into the README or +crate docs. + +## How many threads will Rayon spawn? + +By default, Rayon uses the same number of threads as the number of +CPUs available. Note that on systems with hyperthreading enabled this +equals the number of logical cores and not the physical ones. + +If you want to alter the number of threads spawned, you can set the +environmental variable `RAYON_NUM_THREADS` to the desired number of +threads or use the +[`ThreadPoolBuilder::build_global` function](https://docs.rs/rayon/*/rayon/struct.ThreadPoolBuilder.html#method.build_global) +method. + +## How does Rayon balance work between threads? + +Behind the scenes, Rayon uses a technique called **work stealing** to +try and dynamically ascertain how much parallelism is available and +exploit it. The idea is very simple: we always have a pool of worker +threads available, waiting for some work to do. When you call `join` +the first time, we shift over into that pool of threads. But if you +call `join(a, b)` from a worker thread W, then W will place `b` into +its work queue, advertising that this is work that other worker +threads might help out with. W will then start executing `a`. + +While W is busy with `a`, other threads might come along and take `b` +from its queue. That is called *stealing* `b`. Once `a` is done, W +checks whether `b` was stolen by another thread and, if not, executes +`b` itself. If W runs out of jobs in its own queue, it will look +through the other threads' queues and try to steal work from them. + +This technique is not new. It was first introduced by the +[Cilk project][cilk], done at MIT in the late nineties. The name Rayon +is an homage to that work. + +[cilk]: http://supertech.csail.mit.edu/cilk/ + +## What should I do if I use `Rc`, `Cell`, `RefCell` or other non-Send-and-Sync types? + +There are a number of non-threadsafe types in the Rust standard library, +and if your code is using them, you will not be able to combine it +with Rayon. Similarly, even if you don't have such types, but you try +to have multiple closures mutating the same state, you will get +compilation errors; for example, this function won't work, because +both closures access `slice`: + +```rust +/// Increment all values in slice. +fn increment_all(slice: &mut [i32]) { + rayon::join(|| process(slice), || process(slice)); +} +``` + +The correct way to resolve such errors will depend on the case. Some +cases are easy: for example, uses of [`Rc`] can typically be replaced +with [`Arc`], which is basically equivalent, but thread-safe. + +Code that uses `Cell` or `RefCell`, however, can be somewhat more complicated. +If you can refactor your code to avoid those types, that is often the best way +forward, but otherwise, you can try to replace those types with their threadsafe +equivalents: + +- `Cell` -- replacement: `AtomicUsize`, `AtomicBool`, etc +- `RefCell` -- replacement: `RwLock`, or perhaps `Mutex` + +However, you have to be wary! The parallel versions of these types +have different atomicity guarantees. For example, with a `Cell`, you +can increment a counter like so: + +```rust +let value = counter.get(); +counter.set(value + 1); +``` + +But when you use the equivalent `AtomicUsize` methods, you are +actually introducing a potential race condition (not a data race, +technically, but it can be an awfully fine distinction): + +```rust +let value = tscounter.load(Ordering::SeqCst); +tscounter.store(value + 1, Ordering::SeqCst); +``` + +You can already see that the `AtomicUsize` API is a bit more complex, +as it requires you to specify an +[ordering](http://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html). (I +won't go into the details on ordering here, but suffice to say that if +you don't know what an ordering is, and probably even if you do, you +should use `Ordering::SeqCst`.) The danger in this parallel version of +the counter is that other threads might be running at the same time +and they could cause our counter to get out of sync. For example, if +we have two threads, then they might both execute the "load" before +either has a chance to execute the "store": + +``` +Thread 1 Thread 2 +let value = tscounter.load(Ordering::SeqCst); +// value = X let value = tscounter.load(Ordering::SeqCst); + // value = X +tscounter.store(value+1); tscounter.store(value+1); +// tscounter = X+1 // tscounter = X+1 +``` + +Now even though we've had two increments, we'll only increase the +counter by one! Even though we've got no data race, this is still +probably not the result we wanted. The problem here is that the `Cell` +API doesn't make clear the scope of a "transaction" -- that is, the +set of reads/writes that should occur atomically. In this case, we +probably wanted the get/set to occur together. + +In fact, when using the `Atomic` types, you very rarely want a plain +`load` or plain `store`. You probably want the more complex +operations. A counter, for example, would use `fetch_add` to +atomically load and increment the value in one step. Compare-and-swap +is another popular building block. + +A similar problem can arise when converting `RefCell` to `RwLock`, but +it is somewhat less likely, because the `RefCell` API does in fact +have a notion of a transaction: the scope of the handle returned by +`borrow` or `borrow_mut`. So if you convert each call to `borrow` to +`read` (and `borrow_mut` to `write`), things will mostly work fine in +a parallel setting, but there can still be changes in behavior. +Consider using a `handle: RefCell<Vec<i32>>` like: + +```rust +let len = handle.borrow().len(); +for i in 0 .. len { + let data = handle.borrow()[i]; + println!("{}", data); +} +``` + +In sequential code, we know that this loop is safe. But if we convert +this to parallel code with an `RwLock`, we do not: this is because +another thread could come along and do +`handle.write().unwrap().pop()`, and thus change the length of the +vector. In fact, even in *sequential* code, using very small borrow +sections like this is an anti-pattern: you ought to be enclosing the +entire transaction together, like so: + +```rust +let vec = handle.borrow(); +let len = vec.len(); +for i in 0 .. len { + let data = vec[i]; + println!("{}", data); +} +``` + +Or, even better, using an iterator instead of indexing: + +```rust +let vec = handle.borrow(); +for data in vec { + println!("{}", data); +} +``` + +There are several reasons to prefer one borrow over many. The most +obvious is that it is more efficient, since each borrow has to perform +some safety checks. But it's also more reliable: suppose we modified +the loop above to not just print things out, but also call into a +helper function: + +```rust +let vec = handle.borrow(); +for data in vec { + helper(...); +} +``` + +And now suppose, independently, this helper fn evolved and had to pop +something off of the vector: + +```rust +fn helper(...) { + handle.borrow_mut().pop(); +} +``` + +Under the old model, where we did lots of small borrows, this would +yield precisely the same error that we saw in parallel land using an +`RwLock`: the length would be out of sync and our indexing would fail +(note that in neither case would there be an actual *data race* and +hence there would never be undefined behavior). But now that we use a +single borrow, we'll see a borrow error instead, which is much easier +to diagnose, since it occurs at the point of the `borrow_mut`, rather +than downstream. Similarly, if we move to an `RwLock`, we'll find that +the code either deadlocks (if the write is on the same thread as the +read) or, if the write is on another thread, works just fine. Both of +these are preferable to random failures in my experience. + +## But wait, isn't Rust supposed to free me from this kind of thinking? + +You might think that Rust is supposed to mean that you don't have to +think about atomicity at all. In fact, if you avoid interior +mutability (`Cell` and `RefCell` in a sequential setting, or +`AtomicUsize`, `RwLock`, `Mutex`, et al. in parallel code), then this +is true: the type system will basically guarantee that you don't have +to think about atomicity at all. But often there are times when you +WANT threads to interleave in the ways I showed above. + +Consider for example when you are conducting a search in parallel, say +to find the shortest route. To avoid fruitless search, you might want +to keep a cell with the shortest route you've found thus far. This +way, when you are searching down some path that's already longer than +this shortest route, you can just stop and avoid wasted effort. In +sequential land, you might model this "best result" as a shared value +like `Rc<Cell<usize>>` (here the `usize` represents the length of best +path found so far); in parallel land, you'd use a `Arc<AtomicUsize>`. +Now we can make our search function look like: + +```rust +fn search(path: &Path, cost_so_far: usize, best_cost: &Arc<AtomicUsize>) { + if cost_so_far >= best_cost.load(Ordering::SeqCst) { + return; + } + ... + best_cost.store(...); +} +``` + +Now in this case, we really WANT to see results from other threads +interjected into our execution! |