style: Make style parallel traversal more tunable at runtime

This adds two prefs to configure the parallel traversal work item size
and kick-off threshold, but otherwise shouldn't change behavior.

I switched from iterator generics to just a slice while at it, mostly
for simplicity, but there is a trade-off:

  * When switching from sequential to parallel traversal, we potentially
    pay the price of memmoving the VecDeque around once to make them a
    contiguous slice.

  * However we win in the common case of the smaller-than-work-unit size
    in which case we no longer need to copy stuff to a WorkItem. So I
    think overall this should be an improvement.

Differential Revision: https://phabricator.services.mozilla.com/D178656

2 files changed, 89 insertions, 88 deletions

diff --git a/components/style/driver.rs b/components/style/driver.rs
index b7e33454f41..c02924b73b3 100644
--- a/components/style/driver.rs
+++ b/components/style/driver.rs
@@ -11,7 +11,7 @@ use crate::context::{PerThreadTraversalStatistics, StyleContext};
 use crate::context::{ThreadLocalStyleContext, TraversalStatistics};
 use crate::dom::{SendNode, TElement, TNode};
 use crate::parallel;
-use crate::parallel::{DispatchMode, WORK_UNIT_MAX};
+use crate::parallel::{work_unit_max, DispatchMode};
 use crate::scoped_tls::ScopedTLS;
 use crate::traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
 use rayon;
@@ -48,17 +48,19 @@ fn report_statistics(stats: &PerThreadTraversalStatistics) {
     gecko_stats.mStylesReused += stats.styles_reused;
 }
 
-/// Do a DOM traversal for top-down and (optionally) bottom-up processing,
-/// generic over `D`.
+fn parallelism_threshold() -> usize {
+    static_prefs::pref!("layout.css.stylo-parallelism-threshold") as usize
+}
+
+/// Do a DOM traversal for top-down and (optionally) bottom-up processing, generic over `D`.
 ///
-/// We use an adaptive traversal strategy. We start out with simple sequential
-/// processing, until we arrive at a wide enough level in the DOM that the
-/// parallel traversal would parallelize it. If a thread pool is provided, we
-/// then transfer control over to the parallel traversal.
+/// We use an adaptive traversal strategy. We start out with simple sequential processing, until we
+/// arrive at a wide enough level in the DOM that the parallel traversal would parallelize it.
+/// If a thread pool is provided, we then transfer control over to the parallel traversal.
 ///
-/// Returns true if the traversal was parallel, and also returns the statistics
-/// object containing information on nodes traversed (on nightly only). Not
-/// all of its fields will be initialized since we don't call finish().
+/// Returns true if the traversal was parallel, and also returns the statistics object containing
+/// information on nodes traversed (on nightly only). Not all of its fields will be initialized
+/// since we don't call finish().
 pub fn traverse_dom<E, D>(
     traversal: &D,
     token: PreTraverseToken<E>,
@@ -100,7 +102,9 @@ where
     // Process the nodes breadth-first, just like the parallel traversal does.
     // This helps keep similar traversal characteristics for the style sharing
     // cache.
-    let mut discovered = VecDeque::<SendNode<E::ConcreteNode>>::with_capacity(WORK_UNIT_MAX * 2);
+    let work_unit_max = work_unit_max();
+    let parallelism_threshold = parallelism_threshold();
+    let mut discovered = VecDeque::<SendNode<E::ConcreteNode>>::with_capacity(work_unit_max * 2);
     let mut depth = root.depth();
     let mut nodes_remaining_at_current_depth = 1;
     discovered.push_back(unsafe { SendNode::new(root.as_node()) });
@@ -122,45 +126,48 @@ where
         );
 
         nodes_remaining_at_current_depth -= 1;
-        if nodes_remaining_at_current_depth == 0 {
-            depth += 1;
-            // If there is enough work to parallelize over, and the caller allows
-            // parallelism, switch to the parallel driver. We do this only when
-            // moving to the next level in the dom so that we can pass the same
-            // depth for all the children.
-            if pool.is_some() && discovered.len() > WORK_UNIT_MAX {
-                let pool = pool.unwrap();
-                let tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
-                let root_opaque = root.as_node().opaque();
-                let drain = discovered.drain(..);
-                pool.scope_fifo(|scope| {
-                    // Enable a breadth-first rayon traversal. This causes the work
-                    // queue to be always FIFO, rather than FIFO for stealers and
-                    // FILO for the owner (which is what rayon does by default). This
-                    // ensures that we process all the elements at a given depth before
-                    // proceeding to the next depth, which is important for style sharing.
-                    #[cfg(feature = "gecko")]
-                    gecko_profiler_label!(Layout, StyleComputation);
-                    parallel::traverse_nodes(
-                        drain,
-                        DispatchMode::TailCall,
-                        /* recursion_ok = */ true,
-                        root_opaque,
-                        PerLevelTraversalData {
-                            current_dom_depth: depth,
-                        },
-                        scope,
-                        pool,
-                        traversal,
-                        &tls,
-                    );
-                });
-
-                tls_slots = Some(tls.into_slots());
-                break;
-            }
-            nodes_remaining_at_current_depth = discovered.len();
+
+        // If there is enough work to parallelize over, and the caller allows parallelism, switch
+        // to the parallel driver. We do this only when moving to the next level in the dom so that
+        // we can pass the same depth for all the children.
+        if nodes_remaining_at_current_depth != 0 {
+            continue;
+        }
+        depth += 1;
+        if pool.is_some() &&
+            discovered.len() > parallelism_threshold &&
+            parallelism_threshold > 0
+        {
+            let pool = pool.unwrap();
+            let tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
+            let root_opaque = root.as_node().opaque();
+            pool.scope_fifo(|scope| {
+                // Enable a breadth-first rayon traversal. This causes the work
+                // queue to be always FIFO, rather than FIFO for stealers and
+                // FILO for the owner (which is what rayon does by default). This
+                // ensures that we process all the elements at a given depth before
+                // proceeding to the next depth, which is important for style sharing.
+                #[cfg(feature = "gecko")]
+                gecko_profiler_label!(Layout, StyleComputation);
+                parallel::traverse_nodes(
+                    discovered.make_contiguous(),
+                    DispatchMode::TailCall,
+                    /* recursion_ok = */ true,
+                    root_opaque,
+                    PerLevelTraversalData {
+                        current_dom_depth: depth,
+                    },
+                    scope,
+                    pool,
+                    traversal,
+                    &tls,
+                );
+            });
+
+            tls_slots = Some(tls.into_slots());
+            break;
         }
+        nodes_remaining_at_current_depth = discovered.len();
     }
 
     // Collect statistics from thread-locals if requested.
diff --git a/components/style/parallel.rs b/components/style/parallel.rs
index e578c6bbe51..815393d050d 100644
--- a/components/style/parallel.rs
+++ b/components/style/parallel.rs
@@ -26,8 +26,6 @@ use crate::context::{StyleContext, ThreadLocalStyleContext};
 use crate::dom::{OpaqueNode, SendNode, TElement};
 use crate::scoped_tls::ScopedTLS;
 use crate::traversal::{DomTraversal, PerLevelTraversalData};
-use arrayvec::ArrayVec;
-use itertools::Itertools;
 use rayon;
 use smallvec::SmallVec;
 
@@ -59,23 +57,10 @@ pub const STYLE_THREAD_STACK_SIZE_KB: usize = 512;
 ///
 pub const STACK_SAFETY_MARGIN_KB: usize = 168;
 
-/// The maximum number of child nodes that we will process as a single unit.
-///
-/// Larger values will increase style sharing cache hits and general DOM
-/// locality at the expense of decreased opportunities for parallelism.  There
-/// are some measurements in
-/// https://bugzilla.mozilla.org/show_bug.cgi?id=1385982#c11 and comments 12
-/// and 13 that investigate some slightly different values for the work unit
-/// size.  If the size is significantly increased, make sure to adjust the
-/// condition for kicking off a new work unit in top_down_dom, because
-/// otherwise we're likely to end up doing too much work serially.  For
-/// example, the condition there could become some fraction of WORK_UNIT_MAX
-/// instead of WORK_UNIT_MAX.
-pub const WORK_UNIT_MAX: usize = 16;
-
-/// A set of nodes, sized to the work unit. This gets copied when sent to other
-/// threads, so we keep it compact.
-type WorkUnit<N> = ArrayVec<SendNode<N>, WORK_UNIT_MAX>;
+/// See documentation of the pref for performance characteristics.
+pub fn work_unit_max() -> usize {
+    static_prefs::pref!("layout.css.stylo-work-unit-size") as usize
+}
 
 /// 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
@@ -115,14 +100,15 @@ fn top_down_dom<'a, 'scope, E, D>(
     E: TElement + 'scope,
     D: DomTraversal<E>,
 {
-    debug_assert!(nodes.len() <= WORK_UNIT_MAX);
+    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
+    // 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();
@@ -171,19 +157,19 @@ fn top_down_dom<'a, 'scope, E, D>(
             // 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
+            // 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 {
+            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.drain(..),
+                    &discovered_child_nodes,
                     DispatchMode::NotTailCall,
                     recursion_ok,
                     root,
@@ -193,6 +179,7 @@ fn top_down_dom<'a, 'scope, E, D>(
                     traversal,
                     tls,
                 );
+                discovered_child_nodes.clear();
             }
 
             let node = **n;
@@ -213,7 +200,7 @@ fn top_down_dom<'a, 'scope, E, D>(
     if !discovered_child_nodes.is_empty() {
         traversal_data.current_dom_depth += 1;
         traverse_nodes(
-            discovered_child_nodes.drain(..),
+            &discovered_child_nodes,
             DispatchMode::TailCall,
             recursion_ok,
             root,
@@ -245,8 +232,8 @@ impl DispatchMode {
 /// Enqueues |nodes| for processing, possibly on this thread if the tail call
 /// conditions are met.
 #[inline]
-pub fn traverse_nodes<'a, 'scope, E, D, I>(
-    nodes: I,
+pub fn traverse_nodes<'a, 'scope, E, D>(
+    nodes: &[SendNode<E::ConcreteNode>],
     mode: DispatchMode,
     recursion_ok: bool,
     root: OpaqueNode,
@@ -258,7 +245,6 @@ pub fn traverse_nodes<'a, 'scope, E, D, I>(
 ) where
     E: TElement + 'scope,
     D: DomTraversal<E>,
-    I: ExactSizeIterator<Item = SendNode<E::ConcreteNode>>,
 {
     debug_assert_ne!(nodes.len(), 0);
 
@@ -272,29 +258,37 @@ pub fn traverse_nodes<'a, 'scope, E, D, I>(
     let may_dispatch_tail =
         mode.is_tail_call() && recursion_ok && !pool.current_thread_has_pending_tasks().unwrap();
 
-    // In the common case, our children fit within a single work unit, in which
-    // case we can pass the SmallVec directly and avoid extra allocation.
-    if nodes.len() <= WORK_UNIT_MAX {
-        let work: WorkUnit<E::ConcreteNode> = nodes.collect();
+    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(&work, root, traversal_data, scope, pool, traversal, tls);
+            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);
-                let work = work;
                 top_down_dom(&work, root, traversal_data, scope, pool, traversal, tls);
             });
         }
     } else {
-        for chunk in nodes.chunks(WORK_UNIT_MAX).into_iter() {
-            let nodes: WorkUnit<E::ConcreteNode> = chunk.collect();
+        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 n = nodes;
-                top_down_dom(&*n, root, traversal_data_copy, scope, pool, traversal, tls)
+                let work = work;
+                top_down_dom(
+                    &work,
+                    root,
+                    traversal_data_copy,
+                    scope,
+                    pool,
+                    traversal,
+                    tls,
+                )
             });
         }
     }