# [−][src]Struct rayon_core::ThreadPoolBuilder

    pub struct ThreadPoolBuilder<S = DefaultSpawn> { /* fields omitted */ }

Used to create a new `ThreadPool` or to configure the global rayon thread pool.

## Creating a ThreadPool

The following creates a thread pool with 22 threads.

    let pool = rayon::ThreadPoolBuilder::new().num_threads(22).build().unwrap();

To instead configure the global thread pool, use `build_global()`:

    rayon::ThreadPoolBuilder::new().num_threads(22).build_global().unwrap();

## Methods

### `impl ThreadPoolBuilder`[src]

#### `pub fn new() -> Self`[src]

Creates and returns a valid rayon thread pool builder, but does not initialize it.

### `impl<S> ThreadPoolBuilder<S> where
S: ThreadSpawn,` [src]

Note: the `S: ThreadSpawn` constraint is an internal implementation detail for the default spawn and those set by `spawn_handler`.

#### `pub fn build(self) -> Result<ThreadPool, ThreadPoolBuildError>`[src]

Create a new `ThreadPool` initialized using this configuration.

#### `pub fn build_global(self) -> Result<(), ThreadPoolBuildError>`[src]

Initializes the global thread pool. This initialization is optional. If you do not call this function, the thread pool will be automatically initialized with the default configuration. Calling `build_global` is not recommended, except in two scenarios:

- You wish to change the default configuration.
- You are running a benchmark, in which case initializing may yield slightly more consistent results, since the worker threads will already be ready to go even in the first iteration. But this cost is minimal.

Initialization of the global thread pool happens exactly once. Once started, the configuration cannot be changed. Therefore, if you call `build_global` a second time, it will return an error. An `Ok` result indicates that this is the first initialization of the thread pool.

### `impl ThreadPoolBuilder`[src]

#### `pub fn build_scoped<W, F, R>(
self,
wrapper: W,
with_pool: F
) -> Result<R, ThreadPoolBuildError> where
W: Fn(ThreadBuilder) + Sync,
F: FnOnce(&ThreadPool) -> R,` [src]

Create a scoped `ThreadPool` initialized using this configuration.

This is a convenience function for building a pool using `crossbeam::scope` to spawn threads in a `spawn_handler`. The threads in this pool will start by calling `wrapper`, which should do initialization and continue by calling `ThreadBuilder::run()`.

# Examples

A scoped pool may be useful in combination with scoped thread-local variables.

    #[macro_use]
    extern crate scoped_tls;

    scoped_thread_local!(static POOL_DATA: Vec<i32>);

    fn main() -> Result<(), rayon::ThreadPoolBuildError> {
        let pool_data = vec![1, 2, 3];

        // We haven't assigned any TLS data yet.
        assert!(!POOL_DATA.is_set());

        rayon::ThreadPoolBuilder::new()
            .build_scoped(
                // Borrow `pool_data` in TLS for each thread.
                |thread| POOL_DATA.set(&pool_data, || thread.run()),
                // Do some work that needs the TLS data.
                |pool| pool.install(|| assert!(POOL_DATA.is_set())),
            )?;

        // Once we've returned, `pool_data` is no longer borrowed.
        drop(pool_data);
        Ok(())
    }

### `impl<S> ThreadPoolBuilder<S>`[src]

#### `pub fn spawn_handler<F>(self, spawn: F) -> ThreadPoolBuilder<CustomSpawn<F>> where
F: FnMut(ThreadBuilder) -> Result<()>,` [src]

Set a custom function for spawning threads.

Note that the threads will not exit until after the pool is dropped. It is up to the caller to wait for thread termination if that is important for any invariants. For instance, threads created in `crossbeam::scope` will be joined before that scope returns, and this will block indefinitely if the pool is leaked. Furthermore, the global thread pool doesn't terminate until the entire process exits!

# Examples

A minimal spawn handler just needs to call `run()` from an independent thread.

    fn main() -> Result<(), rayon::ThreadPoolBuildError> {
        let pool = rayon::ThreadPoolBuilder::new()
            .spawn_handler(|thread| {
                std::thread::spawn(|| thread.run());
                Ok(())
            })
            .build()?;

        pool.install(|| println!("Hello from my custom thread!"));
        Ok(())
    }

The default spawn handler sets the name and stack size if given, and propagates any errors from the thread builder.

    fn main() -> Result<(), rayon::ThreadPoolBuildError> {
        let pool = rayon::ThreadPoolBuilder::new()
            .spawn_handler(|thread| {
                let mut b = std::thread::Builder::new();
                if let Some(name) = thread.name() {
                    b = b.name(name.to_owned());
                }
                if let Some(stack_size) = thread.stack_size() {
                    b = b.stack_size(stack_size);
                }
                b.spawn(|| thread.run())?;
                Ok(())
            })
            .build()?;

        pool.install(|| println!("Hello from my fully custom thread!"));
        Ok(())
    }

#### `pub fn thread_name<F>(self, closure: F) -> Self where
F: FnMut(usize) -> String + 'static,` [src]

Set a closure which takes a thread index and returns the thread's name.

#### `pub fn num_threads(self, num_threads: usize) -> Self`[src]

Set the number of threads to be used in the rayon threadpool.

If you specify a non-zero number of threads using this function, then the resulting thread-pools are guaranteed to start at most this number of threads.

If `num_threads` is 0, or you do not call this function, then the Rayon runtime will select the number of threads automatically. At present, this is based on the `RAYON_NUM_THREADS` environment variable (if set), or the number of logical CPUs (otherwise). In the future, however, the default behavior may change to dynamically add or remove threads as needed.

Future compatibility warning: Given the default behavior may change in the future, if you wish to rely on a fixed number of threads, you should use this function to specify that number. To reproduce the current default behavior, you may wish to use the `num_cpus` crate to query the number of CPUs dynamically.

Old environment variable: `RAYON_NUM_THREADS` is a one-to-one replacement of the now deprecated `RAYON_RS_NUM_CPUS` environment variable. If both variables are specified, `RAYON_NUM_THREADS` will be prefered.

#### `pub fn panic_handler<H>(self, panic_handler: H) -> Self where
H: Fn(Box<dyn Any + Send>) + Send + Sync + 'static,` [src]

Normally, whenever Rayon catches a panic, it tries to propagate it to someplace sensible, to try and reflect the semantics of sequential execution. But in some cases, particularly with the `spawn()` APIs, there is no obvious place where we should propagate the panic to. In that case, this panic handler is invoked.

If no panic handler is set, the default is to abort the process, under the principle that panics should not go unobserved.

If the panic handler itself panics, this will abort the process. To prevent this, wrap the body of your panic handler in a call to `std::panic::catch_unwind()`.

#### `pub fn stack_size(self, stack_size: usize) -> Self`[src]

Set the stack size of the worker threads

#### `pub fn breadth_first(self) -> Self`[src]

Deprecated:

use `scope_fifo` and `spawn_fifo` for similar effect

(DEPRECATED) Suggest to worker threads that they execute spawned jobs in a "breadth-first" fashion.

Typically, when a worker thread is idle or blocked, it will attempt to execute the job from the top of its local deque of work (i.e., the job most recently spawned). If this flag is set to true, however, workers will prefer to execute in a breadth-first fashion -- that is, they will search for jobs at the bottom of their local deque. (At present, workers always steal from the bottom of other worker's deques, regardless of the setting of this flag.)

If you think of the tasks as a tree, where a parent task spawns its children in the tree, then this flag loosely corresponds to doing a breadth-first traversal of the tree, whereas the default would be to do a depth-first traversal.

Note that this is an "execution hint". Rayon's task execution is highly dynamic and the precise order in which independent tasks are executed is not intended to be guaranteed.

This `breadth_first()` method is now deprecated per RFC #1, and in the future its effect may be removed. Consider using `scope_fifo()` for a similar effect.

#### `pub fn start_handler<H>(self, start_handler: H) -> Self where
H: Fn(usize) + Send + Sync + 'static,` [src]

Set a callback to be invoked on thread start.

The closure is passed the index of the thread on which it is invoked. Note that this same closure may be invoked multiple times in parallel. If this closure panics, the panic will be passed to the panic handler. If that handler returns, then startup will continue normally.

#### `pub fn exit_handler<H>(self, exit_handler: H) -> Self where
H: Fn(usize) + Send + Sync + 'static,` [src]

Set a callback to be invoked on thread exit.

The closure is passed the index of the thread on which it is invoked. Note that this same closure may be invoked multiple times in parallel. If this closure panics, the panic will be passed to the panic handler. If that handler returns, then the thread will exit normally.

## Trait Implementations

### `impl Default for ThreadPoolBuilder`[src]

#### `fn default() -> Self`[src]

Returns the "default value" for a type. Read more

### `impl<S> Debug for ThreadPoolBuilder<S>`[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

## Auto Trait Implementations

### `impl<S = DefaultSpawn> !Send for ThreadPoolBuilder<S>`

### `impl<S> Unpin for ThreadPoolBuilder<S> where
S: Unpin,`

### `impl<S = DefaultSpawn> !Sync for ThreadPoolBuilder<S>`

### `impl<S = DefaultSpawn> !UnwindSafe for ThreadPoolBuilder<S>`

### `impl<S = DefaultSpawn> !RefUnwindSafe for ThreadPoolBuilder<S>`

## Blanket Implementations

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> From<T> for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

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