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style: Unify parallel and sequential traversal scheduling

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Use in_place_scope_fifo to spawn work into the thread pool while doing
work in the main thread.

Differential Revision: https://phabricator.services.mozilla.com/D179492
This commit is contained in:
Emilio Cobos Álvarez 2023-06-08 08:29:55 +00:00 • committed by Martin Robinson
parent 7771cf25a8
commit 23d60c2195
5 changed files with 151 additions and 293 deletions
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298
components/style/parallel.rs
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@ -27,7 +27,7 @@ use crate::dom::{OpaqueNode, SendNode, TElement};
use crate::scoped_tls::ScopedTLS;
use crate::traversal::{DomTraversal, PerLevelTraversalData};
use rayon;
use smallvec::SmallVec;
use std::collections::VecDeque;
/// The minimum stack size for a thread in the styling pool, in kilobytes.
#[cfg(feature = "gecko")]
@ -54,17 +54,8 @@ pub const STYLE_THREAD_STACK_SIZE_KB: usize = 512;
///
/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1395708#c15
/// [2] See Gecko bug 1376883 for more discussion on the measurements.
///
pub const STACK_SAFETY_MARGIN_KB: usize = 168;
/// See documentation of the pref for performance characteristics.
pub fn work_unit_max() -> usize {
#[cfg(feature = "gecko")]
return static_prefs::pref!("layout.css.stylo-work-unit-size") as usize;
#[cfg(feature = "servo")]
return 16;
}
/// A callback to create our thread local context. This needs to be
/// out of line so we don't allocate stack space for the entire struct
/// in the caller.
@ -76,223 +67,130 @@ where
*slot = Some(ThreadLocalStyleContext::new());
}
/// A parallel top-down DOM traversal.
///
/// This algorithm traverses the DOM in a breadth-first, top-down manner. The
/// goals are:
/// * Never process a child before its parent (since child style depends on
/// parent style). If this were to happen, the styling algorithm would panic.
/// * Prioritize discovering nodes as quickly as possible to maximize
/// opportunities for parallelism. But this needs to be weighed against
/// styling cousins on a single thread to improve sharing.
/// * Style all the children of a given node (i.e. all sibling nodes) on
/// a single thread (with an upper bound to handle nodes with an
/// abnormally large number of children). This is important because we use
/// a thread-local cache to share styles between siblings.
#[inline(always)]
#[allow(unsafe_code)]
fn top_down_dom<'a, 'scope, E, D>(
nodes: &'a [SendNode<E::ConcreteNode>],
root: OpaqueNode,
mut traversal_data: PerLevelTraversalData,
// Sends one chunk of work to the thread-pool.
fn distribute_one_chunk<'a, 'scope, E, D>(
items: VecDeque<SendNode<E::ConcreteNode>>,
traversal_root: OpaqueNode,
work_unit_max: usize,
traversal_data: PerLevelTraversalData,
scope: &'a rayon::ScopeFifo<'scope>,
pool: &'scope rayon::ThreadPool,
traversal: &'scope D,
tls: &'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>,
) where
E: TElement + 'scope,
D: DomTraversal<E>,
{
let work_unit_max = work_unit_max();
debug_assert!(nodes.len() <= work_unit_max);
// We set this below, when we have a borrow of the thread-local-context
// available.
let recursion_ok;
// Collect all the children of the elements in our work unit. This will
// contain the combined children of up to work_unit_max nodes, which may
// be numerous. As such, we store it in a large SmallVec to minimize heap-
// spilling, and never move it.
let mut discovered_child_nodes = SmallVec::<[SendNode<E::ConcreteNode>; 128]>::new();
{
// Scope the borrow of the TLS so that the borrow is dropped before
// a potential recursive call when we pass TailCall.
let mut tlc = tls.ensure(|slot: &mut Option<ThreadLocalStyleContext<E>>| {
create_thread_local_context(slot)
});
// Check that we're not in danger of running out of stack.
recursion_ok = !tlc.stack_limit_checker.limit_exceeded();
scope.spawn_fifo(move |scope| {
gecko_profiler_label!(Layout, StyleComputation);
let mut tlc = tls.ensure(create_thread_local_context);
let mut context = StyleContext {
shared: traversal.shared_context(),
thread_local: &mut *tlc,
};
for n in nodes {
// If the last node we processed produced children, we may want to
// spawn them off into a work item. We do this at the beginning of
// the loop (rather than at the end) so that we can traverse our
// last bits of work directly on this thread without a spawn call.
//
// This has the important effect of removing the allocation and
// context-switching overhead of the parallel traversal for perfectly
// linear regions of the DOM, i.e.:
//
// <russian><doll><tag><nesting></nesting></tag></doll></russian>
//
// which are not at all uncommon.
//
// There's a tension here between spawning off a work item as soon
// as discovered_child_nodes is nonempty and waiting until we have a
// full work item to do so. The former optimizes for speed of
// discovery (we'll start discovering the kids of the things in
// "nodes" ASAP). The latter gives us better sharing (e.g. we can
// share between cousins much better, because we don't hand them off
// as separate work items, which are likely to end up on separate
// threads) and gives us a chance to just handle everything on this
// thread for small DOM subtrees, as in the linear example above.
//
// There are performance and "number of ComputedValues"
// measurements for various testcases in
// https://bugzilla.mozilla.org/show_bug.cgi?id=1385982#c10 and
// following.
//
// The worst case behavior for waiting until we have a full work
// item is a deep tree which has work_unit_max "linear" branches,
// hence work_unit_max elements at each level. Such a tree would
// end up getting processed entirely sequentially, because we would
// process each level one at a time as a single work unit, whether
// via our end-of-loop tail call or not. If we kicked off a
// traversal as soon as we discovered kids, we would instead
// process such a tree more or less with a thread-per-branch,
// multiplexed across our actual threadpool.
if discovered_child_nodes.len() >= work_unit_max {
let mut traversal_data_copy = traversal_data.clone();
traversal_data_copy.current_dom_depth += 1;
traverse_nodes(
&discovered_child_nodes,
DispatchMode::NotTailCall,
recursion_ok,
root,
traversal_data_copy,
scope,
pool,
traversal,
tls,
);
discovered_child_nodes.clear();
}
let node = **n;
let mut children_to_process = 0isize;
traversal.process_preorder(&traversal_data, &mut context, node, |n| {
children_to_process += 1;
let send_n = unsafe { SendNode::new(n) };
discovered_child_nodes.push(send_n);
});
traversal.handle_postorder_traversal(&mut context, root, node, children_to_process);
}
}
// Handle whatever elements we have queued up but not kicked off traversals
// for yet. If any exist, we can process them (or at least one work unit's
// worth of them) directly on this thread by passing TailCall.
if !discovered_child_nodes.is_empty() {
traversal_data.current_dom_depth += 1;
traverse_nodes(
&discovered_child_nodes,
DispatchMode::TailCall,
recursion_ok,
root,
style_trees(
&mut context,
items,
traversal_root,
work_unit_max,
static_prefs::pref!("layout.css.stylo-local-work-queue.in-worker") as usize,
traversal_data,
scope,
pool,
Some(scope),
traversal,
tls,
Some(tls),
);
}
})
}
/// Controls whether traverse_nodes may make a recursive call to continue
/// doing work, or whether it should always dispatch work asynchronously.
#[derive(Clone, Copy, PartialEq)]
pub enum DispatchMode {
/// This is the last operation by the caller.
TailCall,
/// This is not the last operation by the caller.
NotTailCall,
}
impl DispatchMode {
fn is_tail_call(&self) -> bool {
matches!(*self, DispatchMode::TailCall)
}
}
/// Enqueues |nodes| for processing, possibly on this thread if the tail call
/// conditions are met.
#[inline]
pub fn traverse_nodes<'a, 'scope, E, D>(
nodes: &[SendNode<E::ConcreteNode>],
mode: DispatchMode,
recursion_ok: bool,
root: OpaqueNode,
/// Distributes all items into the thread pool, in `work_unit_max` chunks.
fn distribute_work<'a, 'scope, E, D>(
mut items: VecDeque<SendNode<E::ConcreteNode>>,
traversal_root: OpaqueNode,
work_unit_max: usize,
traversal_data: PerLevelTraversalData,
scope: &'a rayon::ScopeFifo<'scope>,
pool: &'scope rayon::ThreadPool,
traversal: &'scope D,
tls: &'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>,
) where
E: TElement + 'scope,
D: DomTraversal<E>,
{
debug_assert_ne!(nodes.len(), 0);
while items.len() > work_unit_max {
let rest = items.split_off(work_unit_max);
distribute_one_chunk(
items,
traversal_root,
work_unit_max,
traversal_data,
scope,
traversal,
tls,
);
items = rest;
}
distribute_one_chunk(
items,
traversal_root,
work_unit_max,
traversal_data,
scope,
traversal,
tls,
);
}
// This is a tail call from the perspective of the caller. However, we only
// want to actually dispatch the job as a tail call if there's nothing left
// in our local queue. Otherwise we need to return to it to maintain proper
// breadth-first ordering. We also need to take care to avoid stack
// overflow due to excessive tail recursion. The stack overflow avoidance
// isn't observable to content -- we're still completely correct, just not
// using tail recursion any more. See Gecko bugs 1368302 and 1376883.
let may_dispatch_tail =
mode.is_tail_call() && recursion_ok && !pool.current_thread_has_pending_tasks().unwrap();
/// Processes `discovered` items, possibly spawning work in other threads as needed.
#[inline]
pub fn style_trees<'a, 'scope, E, D>(
context: &mut StyleContext<E>,
mut discovered: VecDeque<SendNode<E::ConcreteNode>>,
traversal_root: OpaqueNode,
work_unit_max: usize,
local_queue_size: usize,
mut traversal_data: PerLevelTraversalData,
scope: Option<&'a rayon::ScopeFifo<'scope>>,
traversal: &'scope D,
tls: Option<&'scope ScopedTLS<'scope, ThreadLocalStyleContext<E>>>,
) where
E: TElement + 'scope,
D: DomTraversal<E>,
{
let mut nodes_remaining_at_current_depth = discovered.len();
while let Some(node) = discovered.pop_front() {
let mut children_to_process = 0isize;
traversal.process_preorder(&traversal_data, context, *node, |n| {
children_to_process += 1;
discovered.push_back(unsafe { SendNode::new(n) });
});
let work_unit_max = work_unit_max();
// In the common case, our children fit within a single work unit, in which case we can pass
// the nodes directly and avoid extra allocation.
if nodes.len() <= work_unit_max {
if may_dispatch_tail {
top_down_dom(&nodes, root, traversal_data, scope, pool, traversal, tls);
} else {
let work = nodes.to_vec();
scope.spawn_fifo(move |scope| {
#[cfg(feature = "gecko")]
gecko_profiler_label!(Layout, StyleComputation);
top_down_dom(&work, root, traversal_data, scope, pool, traversal, tls);
});
traversal.handle_postorder_traversal(context, traversal_root, *node, children_to_process);
nodes_remaining_at_current_depth -= 1;
// If we have enough children at the next depth in the DOM, spawn them to a different job
// relatively soon, while keeping always at least `local_queue_size` worth of work for
// ourselves.
let discovered_children = discovered.len() - nodes_remaining_at_current_depth;
if discovered_children >= work_unit_max &&
discovered.len() >= local_queue_size + work_unit_max &&
scope.is_some()
{
let kept_work = std::cmp::max(nodes_remaining_at_current_depth, local_queue_size);
let mut traversal_data_copy = traversal_data.clone();
traversal_data_copy.current_dom_depth += 1;
distribute_work(
discovered.split_off(kept_work),
traversal_root,
work_unit_max,
traversal_data_copy,
scope.unwrap(),
traversal,
tls.unwrap(),
);
}
} else {
for chunk in nodes.chunks(work_unit_max) {
let work = chunk.to_vec();
let traversal_data_copy = traversal_data.clone();
scope.spawn_fifo(move |scope| {
#[cfg(feature = "gecko")]
gecko_profiler_label!(Layout, StyleComputation);
let work = work;
top_down_dom(
&work,
root,
traversal_data_copy,
scope,
pool,
traversal,
tls,
)
});
if nodes_remaining_at_current_depth == 0 {
traversal_data.current_dom_depth += 1;
nodes_remaining_at_current_depth = discovered.len();
}
}
}