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//! An interface for dealing with the kinds of parallel computations involved in
//! `bellman`. It's currently just a thin wrapper around [`rayon`] but may be
//! extended in the future to allow for various parallelism strategies.
#[cfg(feature = "multicore")]
mod implementation {
use std::sync::atomic::{AtomicUsize, Ordering};
use crossbeam_channel::{bounded, Receiver};
use lazy_static::lazy_static;
use log::{error, trace};
use rayon::current_num_threads;
static WORKER_SPAWN_COUNTER: AtomicUsize = AtomicUsize::new(0);
lazy_static! {
// See Worker::compute below for a description of this.
static ref WORKER_SPAWN_MAX_COUNT: usize = current_num_threads() * 4;
}
#[derive(Clone, Default)]
pub struct Worker {}
impl Worker {
pub fn new() -> Worker {
Worker {}
}
pub fn log_num_threads(&self) -> u32 {
log2_floor(current_num_threads())
}
pub fn compute<F, R>(&self, f: F) -> Waiter<R>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let (sender, receiver) = bounded(1);
// We keep track here of how many times spawn has been called.
// It can be called without limit, each time, putting a
// request for a new thread to execute a method on the
// ThreadPool. However, if we allow it to be called without
// limits, we run the risk of memory exhaustion due to limited
// stack space consumed by all of the pending closures to be
// executed.
let previous_count = WORKER_SPAWN_COUNTER.fetch_add(1, Ordering::SeqCst);
// If the number of spawns requested has exceeded the number
// of cores available for processing by some factor (the
// default being 4), instead of requesting that we spawn a new
// thread, we instead execute the closure in the context of a
// scope call (which blocks the current thread) to help clear
// the growing work queue and minimize the chances of memory
// exhaustion.
if previous_count > *WORKER_SPAWN_MAX_COUNT {
let thread_index = rayon::current_thread_index().unwrap_or(0);
rayon::scope(move |_| {
trace!("[{}] switching to scope to help clear backlog [threads: current {}, requested {}]",
thread_index,
current_num_threads(),
WORKER_SPAWN_COUNTER.load(Ordering::SeqCst));
let res = f();
sender.send(res).unwrap();
WORKER_SPAWN_COUNTER.fetch_sub(1, Ordering::SeqCst);
});
} else {
rayon::spawn(move || {
let res = f();
sender.send(res).unwrap();
WORKER_SPAWN_COUNTER.fetch_sub(1, Ordering::SeqCst);
});
}
Waiter { receiver }
}
pub fn scope<'a, F, R>(&self, elements: usize, f: F) -> R
where
F: FnOnce(&rayon::Scope<'a>, usize) -> R + Send,
R: Send,
{
let num_threads = current_num_threads();
let chunk_size = if elements < num_threads {
1
} else {
elements / num_threads
};
rayon::scope(|scope| f(scope, chunk_size))
}
}
pub struct Waiter<T> {
receiver: Receiver<T>,
}
impl<T> Waiter<T> {
/// Consumes this waiter and blocks until the result is ready.
pub fn wait(self) -> T {
// This will be Some if this thread is in the global thread pool.
if rayon::current_thread_index().is_some() {
let msg = "wait() cannot be called from within a thread pool since that would lead to deadlocks";
// panic! doesn't necessarily kill the process, so we log as well.
error!("{}", msg);
panic!("{}", msg);
}
self.receiver.recv().unwrap()
}
/// One-off sending.
pub fn done(val: T) -> Self {
let (sender, receiver) = bounded(1);
sender.send(val).unwrap();
Waiter { receiver }
}
}
fn log2_floor(num: usize) -> u32 {
assert!(num > 0);
let mut pow = 0;
while (1 << (pow + 1)) <= num {
pow += 1;
}
pow
}
#[test]
fn test_log2_floor() {
assert_eq!(log2_floor(1), 0);
assert_eq!(log2_floor(2), 1);
assert_eq!(log2_floor(3), 1);
assert_eq!(log2_floor(4), 2);
assert_eq!(log2_floor(5), 2);
assert_eq!(log2_floor(6), 2);
assert_eq!(log2_floor(7), 2);
assert_eq!(log2_floor(8), 3);
}
}
#[cfg(not(feature = "multicore"))]
mod implementation {
#[derive(Clone)]
pub struct Worker;
impl Worker {
pub fn new() -> Worker {
Worker
}
pub fn log_num_threads(&self) -> u32 {
0
}
pub fn compute<F, R>(&self, f: F) -> Waiter<R>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
Waiter::done(f())
}
pub fn scope<F, R>(&self, elements: usize, f: F) -> R
where
F: FnOnce(&DummyScope, usize) -> R,
{
f(&DummyScope, elements)
}
}
pub struct Waiter<T> {
val: Option<T>,
}
impl<T> Waiter<T> {
/// Consumes this waiter and blocks until the result is ready.
pub fn wait(mut self) -> T {
self.val.take().expect("unmet data dependency")
}
/// One-off sending.
pub fn done(val: T) -> Self {
Waiter { val: Some(val) }
}
}
pub struct DummyScope;
impl DummyScope {
pub fn spawn<F: FnOnce(&DummyScope)>(&self, f: F) {
f(self);
}
}
/// A fake rayon ParallelIterator that is just a serial iterator.
pub(crate) trait FakeParallelIterator {
type Iter: Iterator<Item = Self::Item>;
type Item: Send;
fn into_par_iter(self) -> Self::Iter;
}
impl FakeParallelIterator for core::ops::Range<u32> {
type Iter = Self;
type Item = u32;
fn into_par_iter(self) -> Self::Iter {
self
}
}
}
pub use self::implementation::*;