script/
task_queue.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5//! Machinery for [task-queue](https://html.spec.whatwg.org/multipage/#task-queue).
6
7use std::cell::Cell;
8use std::collections::{HashMap, HashSet, VecDeque};
9use std::default::Default;
10
11use base::id::PipelineId;
12use crossbeam_channel::{self, Receiver, Sender};
13use strum::VariantArray;
14
15use crate::dom::bindings::cell::DomRefCell;
16use crate::dom::worker::TrustedWorkerAddress;
17use crate::script_runtime::ScriptThreadEventCategory;
18use crate::task::TaskBox;
19use crate::task_source::TaskSourceName;
20
21pub(crate) type QueuedTask = (
22    Option<TrustedWorkerAddress>,
23    ScriptThreadEventCategory,
24    Box<dyn TaskBox>,
25    Option<PipelineId>,
26    TaskSourceName,
27);
28
29/// Defining the operations used to convert from a msg T to a QueuedTask.
30pub(crate) trait QueuedTaskConversion {
31    fn task_source_name(&self) -> Option<&TaskSourceName>;
32    fn pipeline_id(&self) -> Option<PipelineId>;
33    fn into_queued_task(self) -> Option<QueuedTask>;
34    fn from_queued_task(queued_task: QueuedTask) -> Self;
35    fn inactive_msg() -> Self;
36    fn wake_up_msg() -> Self;
37    fn is_wake_up(&self) -> bool;
38}
39
40pub(crate) struct TaskQueue<T> {
41    /// The original port on which the task-sources send tasks as messages.
42    port: Receiver<T>,
43    /// A sender to ensure the port doesn't block on select while there are throttled tasks.
44    wake_up_sender: Sender<T>,
45    /// A queue from which the event-loop can drain tasks.
46    msg_queue: DomRefCell<VecDeque<T>>,
47    /// A "business" counter, reset for each iteration of the event-loop
48    taken_task_counter: Cell<u64>,
49    /// Tasks that will be throttled for as long as we are "busy".
50    throttled: DomRefCell<HashMap<TaskSourceName, VecDeque<QueuedTask>>>,
51    /// Tasks for not fully-active documents.
52    inactive: DomRefCell<HashMap<PipelineId, VecDeque<QueuedTask>>>,
53}
54
55impl<T: QueuedTaskConversion> TaskQueue<T> {
56    pub(crate) fn new(port: Receiver<T>, wake_up_sender: Sender<T>) -> TaskQueue<T> {
57        TaskQueue {
58            port,
59            wake_up_sender,
60            msg_queue: DomRefCell::new(VecDeque::new()),
61            taken_task_counter: Default::default(),
62            throttled: Default::default(),
63            inactive: Default::default(),
64        }
65    }
66
67    /// Release previously held-back tasks for documents that are now fully-active.
68    /// <https://html.spec.whatwg.org/multipage/#event-loop-processing-model:fully-active>
69    fn release_tasks_for_fully_active_documents(
70        &self,
71        fully_active: &HashSet<PipelineId>,
72    ) -> Vec<T> {
73        self.inactive
74            .borrow_mut()
75            .iter_mut()
76            .filter(|(pipeline_id, _)| fully_active.contains(pipeline_id))
77            .flat_map(|(_, inactive_queue)| {
78                inactive_queue
79                    .drain(0..)
80                    .map(|queued_task| T::from_queued_task(queued_task))
81            })
82            .collect()
83    }
84
85    /// Hold back tasks for currently not fully-active documents.
86    /// <https://html.spec.whatwg.org/multipage/#event-loop-processing-model:fully-active>
87    fn store_task_for_inactive_pipeline(&self, msg: T, pipeline_id: &PipelineId) {
88        let mut inactive = self.inactive.borrow_mut();
89        let inactive_queue = inactive.entry(*pipeline_id).or_default();
90        inactive_queue.push_back(
91            msg.into_queued_task()
92                .expect("Incoming messages should always be convertible into queued tasks"),
93        );
94        let mut msg_queue = self.msg_queue.borrow_mut();
95        if msg_queue.is_empty() {
96            // Ensure there is at least one message.
97            // Otherwise if the just stored inactive message
98            // was the first and last of this iteration,
99            // it will result in a spurious wake-up of the event-loop.
100            msg_queue.push_back(T::inactive_msg());
101        }
102    }
103
104    /// Process incoming tasks, immediately sending priority ones downstream,
105    /// and categorizing potential throttles.
106    fn process_incoming_tasks(&self, first_msg: T, fully_active: &HashSet<PipelineId>) {
107        // 1. Make any previously stored task from now fully-active document available.
108        let mut incoming = self.release_tasks_for_fully_active_documents(fully_active);
109
110        // 2. Process the first message(artifact of the fact that select always returns a message).
111        if !first_msg.is_wake_up() {
112            incoming.push(first_msg);
113        }
114
115        // 3. Process any other incoming message.
116        while let Ok(msg) = self.port.try_recv() {
117            if !msg.is_wake_up() {
118                incoming.push(msg);
119            }
120        }
121
122        // 4. Filter tasks from non-priority task-sources.
123        // TODO: This can use `extract_if` once that is stabilized.
124        let mut to_be_throttled = Vec::new();
125        let mut index = 0;
126        while index != incoming.len() {
127            index += 1; // By default we go to the next index of the vector.
128
129            let task_source = match incoming[index - 1].task_source_name() {
130                Some(task_source) => task_source,
131                None => continue,
132            };
133
134            match task_source {
135                TaskSourceName::PerformanceTimeline => {
136                    to_be_throttled.push(incoming.remove(index - 1));
137                    index -= 1; // We've removed an element, so the next has the same index.
138                },
139                _ => {
140                    // A task that will not be throttled, start counting "business"
141                    self.taken_task_counter
142                        .set(self.taken_task_counter.get() + 1);
143                },
144            }
145        }
146
147        for msg in incoming {
148            // Always run "update the rendering" tasks,
149            // TODO: fix "fully active" concept for iframes.
150            if let Some(TaskSourceName::Rendering) = msg.task_source_name() {
151                self.msg_queue.borrow_mut().push_back(msg);
152                continue;
153            }
154            if let Some(pipeline_id) = msg.pipeline_id() {
155                if !fully_active.contains(&pipeline_id) {
156                    self.store_task_for_inactive_pipeline(msg, &pipeline_id);
157                    continue;
158                }
159            }
160            // Immediately send non-throttled tasks for processing.
161            self.msg_queue.borrow_mut().push_back(msg);
162        }
163
164        for msg in to_be_throttled {
165            // Categorize tasks per task queue.
166            let (worker, category, boxed, pipeline_id, task_source) = match msg.into_queued_task() {
167                Some(queued_task) => queued_task,
168                None => unreachable!(
169                    "A message to be throttled should always be convertible into a queued task"
170                ),
171            };
172            let mut throttled_tasks = self.throttled.borrow_mut();
173            throttled_tasks.entry(task_source).or_default().push_back((
174                worker,
175                category,
176                boxed,
177                pipeline_id,
178                task_source,
179            ));
180        }
181    }
182
183    /// Reset the queue for a new iteration of the event-loop,
184    /// returning the port about whose readiness we want to be notified.
185    pub(crate) fn select(&self) -> &crossbeam_channel::Receiver<T> {
186        // This is a new iteration of the event-loop, so we reset the "business" counter.
187        self.taken_task_counter.set(0);
188        // We want to be notified when the script-port is ready to receive.
189        // Hence that's the one we need to include in the select.
190        &self.port
191    }
192
193    /// Take a message from the front of the queue, without waiting if empty.
194    pub(crate) fn recv(&self) -> Result<T, ()> {
195        self.msg_queue.borrow_mut().pop_front().ok_or(())
196    }
197
198    /// Take all tasks again and then run `recv()`.
199    pub(crate) fn take_tasks_and_recv(&self, fully_active: &HashSet<PipelineId>) -> Result<T, ()> {
200        self.take_tasks(T::wake_up_msg(), fully_active);
201        self.recv()
202    }
203
204    /// Drain the queue for the current iteration of the event-loop.
205    /// Holding-back throttles above a given high-water mark.
206    pub(crate) fn take_tasks(&self, first_msg: T, fully_active: &HashSet<PipelineId>) {
207        // High-watermark: once reached, throttled tasks will be held-back.
208        const PER_ITERATION_MAX: u64 = 5;
209        // Always first check for new tasks, but don't reset 'taken_task_counter'.
210        self.process_incoming_tasks(first_msg, fully_active);
211        let mut throttled = self.throttled.borrow_mut();
212        let mut throttled_length: usize = throttled.values().map(|queue| queue.len()).sum();
213        let mut task_source_cycler = TaskSourceName::VARIANTS.iter().cycle();
214        // "being busy", is defined as having more than x tasks for this loop's iteration.
215        // As long as we're not busy, and there are throttled tasks left:
216        loop {
217            let max_reached = self.taken_task_counter.get() > PER_ITERATION_MAX;
218            let none_left = throttled_length == 0;
219            match (max_reached, none_left) {
220                (_, true) => break,
221                (true, false) => {
222                    // We have reached the high-watermark for this iteration of the event-loop,
223                    // yet also have throttled messages left in the queue.
224                    // Ensure the select wakes up in the next iteration of the event-loop
225                    let _ = self.wake_up_sender.send(T::wake_up_msg());
226                    break;
227                },
228                (false, false) => {
229                    // Cycle through non-priority task sources, taking one throttled task from each.
230                    let task_source = task_source_cycler.next().unwrap();
231                    let throttled_queue = match throttled.get_mut(task_source) {
232                        Some(queue) => queue,
233                        None => continue,
234                    };
235                    let queued_task = match throttled_queue.pop_front() {
236                        Some(queued_task) => queued_task,
237                        None => continue,
238                    };
239                    let msg = T::from_queued_task(queued_task);
240
241                    // Hold back tasks for currently inactive documents.
242                    if let Some(pipeline_id) = msg.pipeline_id() {
243                        if !fully_active.contains(&pipeline_id) {
244                            self.store_task_for_inactive_pipeline(msg, &pipeline_id);
245                            // Reduce the length of throttles,
246                            // but don't add the task to "msg_queue",
247                            // and neither increment "taken_task_counter".
248                            throttled_length -= 1;
249                            continue;
250                        }
251                    }
252
253                    // Make the task available for the event-loop to handle as a message.
254                    self.msg_queue.borrow_mut().push_back(msg);
255                    self.taken_task_counter
256                        .set(self.taken_task_counter.get() + 1);
257                    throttled_length -= 1;
258                },
259            }
260        }
261    }
262}
script/task_queue.rs

script/
task_queue.rs
