Auto merge of #15535 - froydnj:global-style-thread-pool, r=bholley

geckolib: use a global thread pool for styling

By having a single thread pool, rather than one per document, we use less memory.  This addresses https://bugzilla.mozilla.org/show_bug.cgi?id=1324250.

This may be obvious to an experienced Rust programmer, but I went with raw pointers because trying to use `Option` global variables resulted in complaints about turning on feature flags in nightly Rust.  Since this is for stylo, nightly features are not appropriate here.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- 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/15535)
<!-- Reviewable:end -->

3 files changed, 1 insertions, 41 deletions

diff --git a/components/style/Cargo.toml b/components/style/Cargo.toml
index 563c7ed0967..7576ec4b7ea 100644
--- a/components/style/Cargo.toml
+++ b/components/style/Cargo.toml
@@ -13,7 +13,7 @@ path = "lib.rs"
 doctest = false
 
 [features]
-gecko = ["nsstring_vendor", "num_cpus", "rayon/unstable"]
+gecko = ["nsstring_vendor", "rayon/unstable"]
 use_bindgen = ["bindgen", "regex"]
 servo = ["serde/unstable", "serde", "serde_derive", "heapsize_derive",
          "style_traits/servo", "servo_atoms", "html5ever-atoms",
@@ -57,10 +57,6 @@ servo_url = {path = "../url"}
 time = "0.1"
 unicode-segmentation = "1.0"
 
-[dependencies.num_cpus]
-optional = true
-version = "1.0"
-
 [target.'cfg(windows)'.dependencies]
 kernel32-sys = "0.2"
 
diff --git a/components/style/gecko/data.rs b/components/style/gecko/data.rs
index 4ecfe1df520..89c18fa6a06 100644
--- a/components/style/gecko/data.rs
+++ b/components/style/gecko/data.rs
@@ -11,13 +11,9 @@ use gecko_bindings::bindings::RawServoStyleSet;
 use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::sugar::ownership::{HasBoxFFI, HasFFI, HasSimpleFFI};
 use media_queries::Device;
-use num_cpus;
 use parking_lot::RwLock;
 use properties::ComputedValues;
-use rayon;
-use std::cmp;
 use std::collections::HashMap;
-use std::env;
 use std::sync::Arc;
 use std::sync::mpsc::{Receiver, Sender, channel};
 use stylesheets::Stylesheet;
@@ -48,29 +44,12 @@ pub struct PerDocumentStyleDataImpl {
     /// Unused. Will go away when we actually implement transitions and
     /// animations properly.
     pub expired_animations: Arc<RwLock<HashMap<OpaqueNode, Vec<Animation>>>>,
-
-    /// The worker thread pool.
-    /// FIXME(bholley): This shouldn't be per-document.
-    pub work_queue: Option<rayon::ThreadPool>,
-
-    /// The number of threads of the work queue.
-    pub num_threads: usize,
 }
 
 /// The data itself is an `AtomicRefCell`, which guarantees the proper semantics
 /// and unexpected races while trying to mutate it.
 pub struct PerDocumentStyleData(AtomicRefCell<PerDocumentStyleDataImpl>);
 
-lazy_static! {
-    /// The number of layout threads, computed statically.
-    pub static ref NUM_THREADS: usize = {
-        match env::var("STYLO_THREADS").map(|s| s.parse::<usize>().expect("invalid STYLO_THREADS")) {
-            Ok(num) => num,
-            _ => cmp::max(num_cpus::get() * 3 / 4, 1),
-        }
-    };
-}
-
 impl PerDocumentStyleData {
     /// Create a dummy `PerDocumentStyleData`.
     pub fn new(pres_context: RawGeckoPresContextOwned) -> Self {
@@ -86,14 +65,6 @@ impl PerDocumentStyleData {
             new_animations_receiver: new_anims_receiver,
             running_animations: Arc::new(RwLock::new(HashMap::new())),
             expired_animations: Arc::new(RwLock::new(HashMap::new())),
-            work_queue: if *NUM_THREADS <= 1 {
-                None
-            } else {
-                let configuration =
-                    rayon::Configuration::new().set_num_threads(*NUM_THREADS);
-                rayon::ThreadPool::new(configuration).ok()
-            },
-            num_threads: *NUM_THREADS,
         }))
     }
 
@@ -141,9 +112,3 @@ unsafe impl HasFFI for PerDocumentStyleData {
 }
 unsafe impl HasSimpleFFI for PerDocumentStyleData {}
 unsafe impl HasBoxFFI for PerDocumentStyleData {}
-
-impl Drop for PerDocumentStyleDataImpl {
-    fn drop(&mut self) {
-        let _ = self.work_queue.take();
-    }
-}
diff --git a/components/style/lib.rs b/components/style/lib.rs
index 6954c0fe3e7..d94f5436b51 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -61,7 +61,6 @@ extern crate matches;
 #[cfg(feature = "gecko")] extern crate nsstring_vendor as nsstring;
 extern crate num_integer;
 extern crate num_traits;
-#[cfg(feature = "gecko")] extern crate num_cpus;
 extern crate ordered_float;
 extern crate owning_ref;
 extern crate parking_lot;