style/
parallel.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//! Implements parallel traversal over the DOM tree.
6//!
7//! This traversal is based on Rayon, and therefore its safety is largely
8//! verified by the type system.
9//!
10//! The primary trickiness and fine print for the above relates to the
11//! thread safety of the DOM nodes themselves. Accessing a DOM element
12//! concurrently on multiple threads is actually mostly "safe", since all
13//! the mutable state is protected by an AtomicRefCell, and so we'll
14//! generally panic if something goes wrong. Still, we try to to enforce our
15//! thread invariants at compile time whenever possible. As such, TNode and
16//! TElement are not Send, so ordinary style system code cannot accidentally
17//! share them with other threads. In the parallel traversal, we explicitly
18//! invoke |unsafe { SendNode::new(n) }| to put nodes in containers that may
19//! be sent to other threads. This occurs in only a handful of places and is
20//! easy to grep for. At the time of this writing, there is no other unsafe
21//! code in the parallel traversal.
22
23#![deny(missing_docs)]
24
25use crate::context::{StyleContext, ThreadLocalStyleContext};
26use crate::dom::{OpaqueNode, SendNode, TElement};
27use crate::scoped_tls::ScopedTLS;
28use crate::traversal::{DomTraversal, PerLevelTraversalData};
29use std::collections::VecDeque;
30
31/// The minimum stack size for a thread in the styling pool, in kilobytes.
32#[cfg(feature = "gecko")]
33pub const STYLE_THREAD_STACK_SIZE_KB: usize = 256;
34
35/// The minimum stack size for a thread in the styling pool, in kilobytes.
36/// Servo requires a bigger stack in debug builds.
37#[cfg(feature = "servo")]
38pub const STYLE_THREAD_STACK_SIZE_KB: usize = 512;
39
40/// The stack margin. If we get this deep in the stack, we will skip recursive
41/// optimizations to ensure that there is sufficient room for non-recursive work.
42///
43/// We allocate large safety margins because certain OS calls can use very large
44/// amounts of stack space [1]. Reserving a larger-than-necessary stack costs us
45/// address space, but if we keep our safety margin big, we will generally avoid
46/// committing those extra pages, and only use them in edge cases that would
47/// otherwise cause crashes.
48///
49/// When measured with 128KB stacks and 40KB margin, we could support 53
50/// levels of recursion before the limiter kicks in, on x86_64-Linux [2]. When
51/// we doubled the stack size, we added it all to the safety margin, so we should
52/// be able to get the same amount of recursion.
53///
54/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1395708#c15
55/// [2] See Gecko bug 1376883 for more discussion on the measurements.
56pub const STACK_SAFETY_MARGIN_KB: usize = 168;
57
58/// A callback to create our thread local context.  This needs to be
59/// out of line so we don't allocate stack space for the entire struct
60/// in the caller.
61#[inline(never)]
62pub(crate) fn create_thread_local_context<'scope, E>(slot: &mut Option<ThreadLocalStyleContext<E>>)
63where
64    E: TElement + 'scope,
65{
66    *slot = Some(ThreadLocalStyleContext::new());
67}
68
69// Sends one chunk of work to the thread-pool.
70fn distribute_one_chunk<'a, 'scope, E, D>(
71    items: VecDeque<SendNode<E::ConcreteNode>>,
72    traversal_root: OpaqueNode,
73    work_unit_max: usize,
74    traversal_data: PerLevelTraversalData,
75    scope: &'a rayon::ScopeFifo<'scope>,
76    traversal: &'scope D,
77    tls: &'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>,
78) where
79    E: TElement + 'scope,
80    D: DomTraversal<E>,
81{
82    scope.spawn_fifo(move |scope| {
83        #[cfg(feature = "gecko")]
84        gecko_profiler_label!(Layout, StyleComputation);
85        let mut tlc = tls.ensure(create_thread_local_context);
86        let mut context = StyleContext {
87            shared: traversal.shared_context(),
88            thread_local: &mut *tlc,
89        };
90        style_trees(
91            &mut context,
92            items,
93            traversal_root,
94            work_unit_max,
95            traversal_data,
96            Some(scope),
97            traversal,
98            tls,
99        );
100    })
101}
102
103/// Distributes all items into the thread pool, in `work_unit_max` chunks.
104fn distribute_work<'a, 'scope, E, D>(
105    mut items: impl Iterator<Item = SendNode<E::ConcreteNode>>,
106    traversal_root: OpaqueNode,
107    work_unit_max: usize,
108    traversal_data: PerLevelTraversalData,
109    scope: &'a rayon::ScopeFifo<'scope>,
110    traversal: &'scope D,
111    tls: &'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>,
112) where
113    E: TElement + 'scope,
114    D: DomTraversal<E>,
115{
116    use std::iter::FromIterator;
117    loop {
118        let chunk = VecDeque::from_iter(items.by_ref().take(work_unit_max));
119        if chunk.is_empty() {
120            return;
121        }
122        distribute_one_chunk(
123            chunk,
124            traversal_root,
125            work_unit_max,
126            traversal_data,
127            scope,
128            traversal,
129            tls,
130        );
131    }
132}
133
134/// Processes `discovered` items, possibly spawning work in other threads as needed.
135#[inline]
136pub fn style_trees<'a, 'scope, E, D>(
137    context: &mut StyleContext<E>,
138    mut discovered: VecDeque<SendNode<E::ConcreteNode>>,
139    traversal_root: OpaqueNode,
140    work_unit_max: usize,
141    mut traversal_data: PerLevelTraversalData,
142    scope: Option<&'a rayon::ScopeFifo<'scope>>,
143    traversal: &'scope D,
144    tls: &'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>,
145) where
146    E: TElement + 'scope,
147    D: DomTraversal<E>,
148{
149    let local_queue_size = if tls.current_thread_index() == 0 {
150        static_prefs::pref!("layout.css.stylo-local-work-queue.in-main-thread")
151    } else {
152        static_prefs::pref!("layout.css.stylo-local-work-queue.in-worker")
153    } as usize;
154
155    let mut nodes_remaining_at_current_depth = discovered.len();
156    while let Some(node) = discovered.pop_front() {
157        let mut children_to_process = 0isize;
158        traversal.process_preorder(&traversal_data, context, *node, |n| {
159            children_to_process += 1;
160            discovered.push_back(unsafe { SendNode::new(n) });
161        });
162
163        traversal.handle_postorder_traversal(context, traversal_root, *node, children_to_process);
164
165        nodes_remaining_at_current_depth -= 1;
166
167        // If we have enough children at the next depth in the DOM, spawn them to a different job
168        // relatively soon, while keeping always at least `local_queue_size` worth of work for
169        // ourselves.
170        let discovered_children = discovered.len() - nodes_remaining_at_current_depth;
171        if discovered_children >= work_unit_max
172            && discovered.len() >= local_queue_size + work_unit_max
173            && scope.is_some()
174        {
175            let kept_work = std::cmp::max(nodes_remaining_at_current_depth, local_queue_size);
176            let mut traversal_data_copy = traversal_data.clone();
177            traversal_data_copy.current_dom_depth += 1;
178            distribute_work(
179                discovered.range(kept_work..).cloned(),
180                traversal_root,
181                work_unit_max,
182                traversal_data_copy,
183                scope.unwrap(),
184                traversal,
185                tls,
186            );
187            discovered.truncate(kept_work);
188        }
189
190        if nodes_remaining_at_current_depth == 0 {
191            traversal_data.current_dom_depth += 1;
192            nodes_remaining_at_current_depth = discovered.len();
193        }
194    }
195}
style/parallel.rs

style/
parallel.rs
