style: Make Servo use a single thread-pool for layout-related tasks per-process.

Instead of per-document. This also allows to reuse this thread-pool if needed
for other stuff, like parallel CSS parsing (#22478), and to share more code with
Gecko, which is always nice.

components/style/gecko/global_style_data.rs

@@ -1,117 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
-
-//! Global style data
-
-use crate::context::StyleSystemOptions;
-use crate::gecko_bindings::bindings;
-use crate::parallel::STYLE_THREAD_STACK_SIZE_KB;
-use crate::shared_lock::SharedRwLock;
-use crate::thread_state;
-use num_cpus;
-use rayon;
-use std::cmp;
-use std::env;
-use std::ffi::CString;
-
-/// Global style data
-pub struct GlobalStyleData {
-    /// Shared RWLock for CSSOM objects
-    pub shared_lock: SharedRwLock,
-
-    /// Global style system options determined by env vars.
-    pub options: StyleSystemOptions,
-}
-
-/// Global thread pool
-pub struct StyleThreadPool {
-    /// How many threads parallel styling can use.
-    pub num_threads: usize,
-
-    /// The parallel styling thread pool.
-    pub style_thread_pool: Option<rayon::ThreadPool>,
-}
-
-fn thread_name(index: usize) -> String {
-    format!("StyleThread#{}", index)
-}
-
-fn thread_startup(index: usize) {
-    thread_state::initialize_layout_worker_thread();
-    unsafe {
-        bindings::Gecko_SetJemallocThreadLocalArena(true);
-    }
-    let name = thread_name(index);
-    let name = CString::new(name).unwrap();
-    unsafe {
-        // Gecko_RegisterProfilerThread copies the passed name here.
-        bindings::Gecko_RegisterProfilerThread(name.as_ptr());
-    }
-}
-
-fn thread_shutdown(_: usize) {
-    unsafe {
-        bindings::Gecko_UnregisterProfilerThread();
-        bindings::Gecko_SetJemallocThreadLocalArena(false);
-    }
-}
-
-lazy_static! {
-    /// Global thread pool
-    pub static ref STYLE_THREAD_POOL: StyleThreadPool = {
-        let stylo_threads = env::var("STYLO_THREADS")
-            .map(|s| s.parse::<usize>().expect("invalid STYLO_THREADS value"));
-        let mut num_threads = match stylo_threads {
-            Ok(num) => num,
-            // The default heuristic is num_virtual_cores * .75. This gives us
-            // three threads on a hyper-threaded dual core, and six threads on
-            // a hyper-threaded quad core. The performance benefit of additional
-            // threads seems to level off at around six, so we cap it there on
-            // many-core machines (see bug 1431285 comment 14).
-            _ => cmp::min(cmp::max(num_cpus::get() * 3 / 4, 1), 6),
-        };
-
-        // If num_threads is one, there's no point in creating a thread pool, so
-        // force it to zero.
-        //
-        // We allow developers to force a one-thread pool for testing via a
-        // special environmental variable.
-        if num_threads == 1 {
-            let force_pool = env::var("FORCE_STYLO_THREAD_POOL")
-                .ok().map_or(false, |s| s.parse::<usize>().expect("invalid FORCE_STYLO_THREAD_POOL value") == 1);
-            if !force_pool {
-                num_threads = 0;
-            }
-        }
-
-        let pool = if num_threads < 1 {
-            None
-        } else {
-            let workers = rayon::ThreadPoolBuilder::new()
-                .num_threads(num_threads)
-                // 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.
-                .breadth_first()
-                .thread_name(thread_name)
-                .start_handler(thread_startup)
-                .exit_handler(thread_shutdown)
-                .stack_size(STYLE_THREAD_STACK_SIZE_KB * 1024)
-                .build();
-            workers.ok()
-        };
-
-        StyleThreadPool {
-            num_threads: num_threads,
-            style_thread_pool: pool,
-        }
-    };
-    /// Global style data
-    pub static ref GLOBAL_STYLE_DATA: GlobalStyleData = GlobalStyleData {
-        shared_lock: SharedRwLock::new(),
-        options: StyleSystemOptions::default(),
-    };
-}

components/style/gecko/mod.rs

@@ -10,7 +10,6 @@ mod non_ts_pseudo_class_list;
 pub mod arc_types;
 pub mod conversions;
 pub mod data;
-pub mod global_style_data;
 pub mod media_features;
 pub mod media_queries;
 pub mod pseudo_element;