Auto merge of #12862 - servo:layout-new, r=emilio

added dom obj counting to decide sequential/parallel layout (#10110)

This is a rebased version of #11713

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #10110 (github issue number if applicable).
- [X] There are no tests for these changes because it's an optimization with no visible behavioral changes

<!-- Reviewable:start -->

---

This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12862)

<!-- Reviewable:end -->

components/layout_thread/lib.rs

@@ -175,6 +175,9 @@ pub struct LayoutThread {
     /// The workers that we use for parallel operation.
     parallel_traversal: Option<rayon::ThreadPool>,
 
+    /// Flag to indicate whether to use parallel operations
+    parallel_flag: bool,
+
     /// Starts at zero, and increased by one every time a layout completes.
     /// This can be used to easily check for invalid stale data.
     generation: u32,
@@ -389,13 +392,11 @@ impl LayoutThread {
         let device = Device::new(
             MediaType::Screen,
             opts::get().initial_window_size.to_f32() * ScaleFactor::new(1.0));
-        let parallel_traversal = if layout_threads != 1 {
+
         let configuration =
             rayon::Configuration::new().set_num_threads(layout_threads);
-            rayon::ThreadPool::new(configuration).ok()
-        } else {
-            None
-        };
+        let parallel_traversal = rayon::ThreadPool::new(configuration).ok();
+        debug!("Possible layout Threads: {}", layout_threads);
 
         // Create the channel on which new animations can be sent.
         let (new_animations_sender, new_animations_receiver) = channel();
@@ -441,6 +442,7 @@ impl LayoutThread {
             font_cache_receiver: font_cache_receiver,
             font_cache_sender: ipc_font_cache_sender,
             parallel_traversal: parallel_traversal,
+            parallel_flag: true,
             generation: 0,
             new_animations_sender: new_animations_sender,
             new_animations_receiver: new_animations_receiver,
@@ -979,6 +981,13 @@ impl LayoutThread {
                       (data.reflow_info.goal == ReflowGoal::ForScriptQuery &&
                        data.query_type != ReflowQueryType::NoQuery));
 
+        // Parallelize if there's more than 750 objects based on rzambre's suggestion
+        // https://github.com/servo/servo/issues/10110
+        self.parallel_flag = self.layout_threads > 1 && data.dom_count > 750;
+        debug!("layout: received layout request for: {}", self.url);
+        debug!("Number of objects in DOM: {}", data.dom_count);
+        debug!("layout: parallel? {}", self.parallel_flag);
+
         let mut rw_data = possibly_locked_rw_data.lock();
 
         let element: ServoLayoutElement = match document.root_node() {
@@ -1140,18 +1149,16 @@ impl LayoutThread {
                     self.time_profiler_chan.clone(),
                     || {
                 // Perform CSS selector matching and flow construction.
-                match self.parallel_traversal {
-                    None => {
+                if let (true, Some(traversal)) = (self.parallel_flag, self.parallel_traversal.as_mut()) {
+                    // Parallel mode
+                    parallel::traverse_dom::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
+                        element.as_node(), dom_depth, &shared_layout_context, traversal);
+                } else {
+                    // Sequential mode
                     sequential::traverse_dom::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
                         element.as_node(), &shared_layout_context);
                 }
-                    Some(ref mut traversal) => {
-                        parallel::traverse_dom::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
-                            element.as_node(), dom_depth, &shared_layout_context, traversal);
-                    }
-                }
             });
-
             // TODO(pcwalton): Measure energy usage of text shaping, perhaps?
             let text_shaping_time =
                 (font::get_and_reset_text_shaping_performance_counter() as u64) /
@@ -1402,19 +1409,17 @@ impl LayoutThread {
                         self.time_profiler_chan.clone(),
                         || {
                     let profiler_metadata = self.profiler_metadata();
-                    match self.parallel_traversal {
-                        None => {
-                            // Sequential mode.
-                            LayoutThread::solve_constraints(FlowRef::deref_mut(&mut root_flow), &layout_context)
-                        }
-                        Some(ref mut parallel) => {
+
+                    if let (true, Some(traversal)) = (self.parallel_flag, self.parallel_traversal.as_mut()) {
                         // Parallel mode.
-                            LayoutThread::solve_constraints_parallel(parallel,
+                        LayoutThread::solve_constraints_parallel(traversal,
                                                                  FlowRef::deref_mut(&mut root_flow),
                                                                  profiler_metadata,
                                                                  self.time_profiler_chan.clone(),
                                                                  &*layout_context);
-                        }
+                    } else {
+                        //Sequential mode
+                        LayoutThread::solve_constraints(FlowRef::deref_mut(&mut root_flow), &layout_context)
                     }
                 });
             }

components/script/dom/document.rs

@@ -284,6 +284,12 @@ pub struct Document {
     last_click_info: DOMRefCell<Option<(Instant, Point2D<f32>)>>,
     /// https://html.spec.whatwg.org/multipage/#ignore-destructive-writes-counter
     ignore_destructive_writes_counter: Cell<u32>,
+    /// Track the total number of elements in this DOM's tree.
+    /// This is sent to the layout thread every time a reflow is done;
+    /// layout uses this to determine if the gains from parallel layout will be worth the overhead.
+    ///
+    /// See also: https://github.com/servo/servo/issues/10110
+    dom_count: Cell<u32>,
 }
 
 #[derive(JSTraceable, HeapSizeOf)]
@@ -455,6 +461,22 @@ impl Document {
         self.base_element.set(base.r());
     }
 
+    pub fn dom_count(&self) -> u32 {
+        self.dom_count.get()
+    }
+
+    /// This is called by `bind_to_tree` when a node is added to the DOM.
+    /// The internal count is used by layout to determine whether to be sequential or parallel.
+    /// (it's sequential for small DOMs)
+    pub fn increment_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() + 1);
+    }
+
+    /// This is called by `unbind_from_tree` when a node is removed from the DOM.
+    pub fn decrement_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() - 1);
+    }
+
     pub fn quirks_mode(&self) -> QuirksMode {
         self.quirks_mode.get()
     }
@@ -1884,6 +1906,7 @@ impl Document {
             target_element: MutNullableHeap::new(None),
             last_click_info: DOMRefCell::new(None),
             ignore_destructive_writes_counter: Default::default(),
+            dom_count: Cell::new(1),
         }
     }
 

components/script/dom/element.rs

@@ -2151,10 +2151,12 @@ impl VirtualMethods for Element {
             return;
         }
 
-        if let Some(ref value) = *self.id_attribute.borrow() {
         let doc = document_from_node(self);
+        if let Some(ref value) = *self.id_attribute.borrow() {
             doc.register_named_element(self, value.clone());
         }
+        // This is used for layout optimization.
+        doc.increment_dom_count();
     }
 
     fn unbind_from_tree(&self, context: &UnbindContext) {
@@ -2164,10 +2166,12 @@ impl VirtualMethods for Element {
             return;
         }
 
-        if let Some(ref value) = *self.id_attribute.borrow() {
         let doc = document_from_node(self);
+        if let Some(ref value) = *self.id_attribute.borrow() {
             doc.unregister_named_element(self, value.clone());
         }
+        // This is used for layout optimization.
+        doc.decrement_dom_count();
     }
 
     fn children_changed(&self, mutation: &ChildrenMutation) {

components/script/dom/htmlheadelement.rs

@@ -71,7 +71,10 @@ impl VirtualMethods for HTMLHeadElement {
     fn super_type(&self) -> Option<&VirtualMethods> {
         Some(self.upcast::<HTMLElement>() as &VirtualMethods)
     }
-    fn bind_to_tree(&self, _tree_in_doc: bool) {
+    fn bind_to_tree(&self, tree_in_doc: bool) {
+        if let Some(ref s) = self.super_type() {
+            s.bind_to_tree(tree_in_doc);
+        }
         load_script(self);
     }
 }

components/script/dom/htmltitleelement.rs

@@ -71,9 +71,12 @@ impl VirtualMethods for HTMLTitleElement {
         }
     }
 
-    fn bind_to_tree(&self, is_in_doc: bool) {
+    fn bind_to_tree(&self, tree_in_doc: bool) {
+        if let Some(ref s) = self.super_type() {
+            s.bind_to_tree(tree_in_doc);
+        }
         let node = self.upcast::<Node>();
-        if is_in_doc {
+        if tree_in_doc {
             node.owner_doc().title_changed();
         }
     }

components/script/dom/window.rs

@@ -1084,6 +1084,7 @@ impl Window {
             window_size: window_size,
             script_join_chan: join_chan,
             query_type: query_type,
+            dom_count: self.Document().dom_count(),
         };
 
         self.layout_chan.send(Msg::Reflow(reflow)).unwrap();

components/script_layout_interface/message.rs

@@ -126,6 +126,8 @@ pub struct ScriptReflow {
     pub script_join_chan: Sender<()>,
     /// The type of query if any to perform during this reflow.
     pub query_type: ReflowQueryType,
+    /// The number of objects in the dom #10110
+    pub dom_count: u32,
 }
 
 impl Drop for ScriptReflow {