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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/. */
//! Machinery for [task-queue](https://html.spec.whatwg.org/multipage/#task-queue).
use crate::dom::bindings::cell::DomRefCell;
use crate::dom::worker::TrustedWorkerAddress;
use crate::script_runtime::ScriptThreadEventCategory;
use crate::task::TaskBox;
use crate::task_source::TaskSourceName;
use crossbeam_channel::{self, Receiver, Sender};
use msg::constellation_msg::PipelineId;
use std::cell::Cell;
use std::collections::{HashMap, VecDeque};
use std::default::Default;
pub type QueuedTask = (
Option<TrustedWorkerAddress>,
ScriptThreadEventCategory,
Box<dyn TaskBox>,
Option<PipelineId>,
TaskSourceName,
);
/// Defining the operations used to convert from a msg T to a QueuedTask.
pub trait QueuedTaskConversion {
fn task_source_name(&self) -> Option<&TaskSourceName>;
fn into_queued_task(self) -> Option<QueuedTask>;
fn from_queued_task(queued_task: QueuedTask) -> Self;
fn wake_up_msg() -> Self;
fn is_wake_up(&self) -> bool;
}
pub struct TaskQueue<T> {
/// The original port on which the task-sources send tasks as messages.
port: Receiver<T>,
/// A sender to ensure the port doesn't block on select while there are throttled tasks.
wake_up_sender: Sender<T>,
/// A queue from which the event-loop can drain tasks.
msg_queue: DomRefCell<VecDeque<T>>,
/// A "business" counter, reset for each iteration of the event-loop
taken_task_counter: Cell<u64>,
/// Tasks that will be throttled for as long as we are "busy".
throttled: DomRefCell<HashMap<TaskSourceName, VecDeque<QueuedTask>>>,
}
impl<T: QueuedTaskConversion> TaskQueue<T> {
pub fn new(port: Receiver<T>, wake_up_sender: Sender<T>) -> TaskQueue<T> {
TaskQueue {
port,
wake_up_sender,
msg_queue: DomRefCell::new(VecDeque::new()),
taken_task_counter: Default::default(),
throttled: Default::default(),
}
}
/// Process incoming tasks, immediately sending priority ones downstream,
/// and categorizing potential throttles.
fn process_incoming_tasks(&self, first_msg: T) {
let mut incoming = Vec::with_capacity(self.port.len() + 1);
if !first_msg.is_wake_up() {
incoming.push(first_msg);
}
while let Ok(msg) = self.port.try_recv() {
if !msg.is_wake_up() {
incoming.push(msg);
}
}
let to_be_throttled: Vec<T> = incoming
.drain_filter(|msg| {
let task_source = match msg.task_source_name() {
Some(task_source) => task_source,
None => return false,
};
match task_source {
TaskSourceName::PerformanceTimeline => return true,
_ => {
// A task that will not be throttled, start counting "business"
self.taken_task_counter
.set(self.taken_task_counter.get() + 1);
return false;
},
}
})
.collect();
for msg in incoming {
// Immediately send non-throttled tasks for processing.
let _ = self.msg_queue.borrow_mut().push_back(msg);
}
for msg in to_be_throttled {
// Categorize tasks per task queue.
let (worker, category, boxed, pipeline_id, task_source) = match msg.into_queued_task() {
Some(queued_task) => queued_task,
None => unreachable!(
"A message to be throttled should always be convertible into a queued task"
),
};
let mut throttled_tasks = self.throttled.borrow_mut();
throttled_tasks
.entry(task_source.clone())
.or_insert(VecDeque::new())
.push_back((worker, category, boxed, pipeline_id, task_source));
}
}
/// Reset the queue for a new iteration of the event-loop,
/// returning the port about whose readiness we want to be notified.
pub fn select(&self) -> &crossbeam_channel::Receiver<T> {
// This is a new iteration of the event-loop, so we reset the "business" counter.
self.taken_task_counter.set(0);
// We want to be notified when the script-port is ready to receive.
// Hence that's the one we need to include in the select.
&self.port
}
/// Take a message from the front of the queue, without waiting if empty.
pub fn recv(&self) -> Result<T, ()> {
self.msg_queue.borrow_mut().pop_front().ok_or(())
}
/// Same as recv.
pub fn try_recv(&self) -> Result<T, ()> {
self.recv()
}
/// Drain the queue for the current iteration of the event-loop.
/// Holding-back throttles above a given high-water mark.
pub fn take_tasks(&self, first_msg: T) {
// High-watermark: once reached, throttled tasks will be held-back.
const PER_ITERATION_MAX: u64 = 5;
// Always first check for new tasks, but don't reset 'taken_task_counter'.
self.process_incoming_tasks(first_msg);
let mut throttled = self.throttled.borrow_mut();
let mut throttled_length: usize = throttled.values().map(|queue| queue.len()).sum();
let task_source_names = TaskSourceName::all();
let mut task_source_cycler = task_source_names.iter().cycle();
// "being busy", is defined as having more than x tasks for this loop's iteration.
// As long as we're not busy, and there are throttled tasks left:
loop {
let max_reached = self.taken_task_counter.get() > PER_ITERATION_MAX;
let none_left = throttled_length == 0;
match (max_reached, none_left) {
(_, true) => break,
(true, false) => {
// We have reached the high-watermark for this iteration of the event-loop,
// yet also have throttled messages left in the queue.
// Ensure the select wakes up in the next iteration of the event-loop
let _ = self.wake_up_sender.send(T::wake_up_msg());
break;
},
(false, false) => {
// Cycle through non-priority task sources, taking one throttled task from each.
let task_source = task_source_cycler.next().unwrap();
let throttled_queue = match throttled.get_mut(&task_source) {
Some(queue) => queue,
None => continue,
};
let queued_task = match throttled_queue.pop_front() {
Some(queued_task) => queued_task,
None => continue,
};
let msg = T::from_queued_task(queued_task);
let _ = self.msg_queue.borrow_mut().push_back(msg);
self.taken_task_counter
.set(self.taken_task_counter.get() + 1);
throttled_length = throttled_length - 1;
},
}
}
}
}
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