Improve Cargo's scheduling of builds

src/cargo/ops/cargo_rustc/job_queue.rs

@@ -111,6 +111,7 @@ impl<'a> JobQueue<'a> {
     /// possible along each dependency chain.
     pub fn execute(&mut self, cx: &mut Context) -> CargoResult<()> {
         let _p = profile::start("executing the job graph");
+        self.queue.queue_finished();
 
         // We need to give a handle to the send half of our message queue to the
         // jobserver helper thread. Unfortunately though we need the handle to be

src/cargo/util/dependency_queue.rs

@@ -34,6 +34,9 @@ pub struct DependencyQueue<K: Eq + Hash, V> {
     /// The packages which are currently being built, waiting for a call to
     /// `finish`.
     pending: HashSet<K>,
+
+    /// Topological depth of each key
+    depth: HashMap<K, usize>,
 }
 
 /// Indication of the freshness of a package.
@@ -66,6 +69,7 @@ impl<K: Hash + Eq + Clone, V> DependencyQueue<K, V> {
             reverse_dep_map: HashMap::new(),
             dirty: HashSet::new(),
             pending: HashSet::new(),
+            depth: HashMap::new(),
         }
     }
 
@@ -97,14 +101,60 @@ impl<K: Hash + Eq + Clone, V> DependencyQueue<K, V> {
         &mut slot.insert((my_dependencies, value)).1
     }
 
+    /// All nodes have been added, calculate some internal metadata and prepare
+    /// for `dequeue`.
+    pub fn queue_finished(&mut self) {
+        for key in self.dep_map.keys() {
+            depth(key, &self.reverse_dep_map, &mut self.depth);
+        }
+
+        fn depth<K: Hash + Eq + Clone>(
+            key: &K,
+            map: &HashMap<K, HashSet<K>>,
+            results: &mut HashMap<K, usize>,
+        ) -> usize {
+            const IN_PROGRESS: usize = !0;
+
+            if let Some(&depth) = results.get(key) {
+                assert_ne!(depth, IN_PROGRESS, "cycle in DependencyQueue");
+                return depth;
+            }
+
+            results.insert(key.clone(), IN_PROGRESS);
+
+            let depth = 1 + map.get(&key)
+                .into_iter()
+                .flat_map(|it| it)
+                .map(|dep| depth(dep, map, results))
+                .max()
+                .unwrap_or(0);
+
+            *results.get_mut(key).unwrap() = depth;
+
+            depth
+        }
+    }
+
     /// Dequeues a package that is ready to be built.
     ///
     /// A package is ready to be built when it has 0 un-built dependencies. If
     /// `None` is returned then no packages are ready to be built.
     pub fn dequeue(&mut self) -> Option<(Freshness, K, V)> {
-        let key = match self.dep_map.iter()
-                                    .find(|&(_, &(ref deps, _))| deps.is_empty())
-                                    .map(|(key, _)| key.clone()) {
+        // Look at all our crates and find everything that's ready to build (no
+        // deps). After we've got that candidate set select the one which has
+        // the maximum depth in the dependency graph. This way we should
+        // hopefully keep CPUs hottest the longest by ensuring that long
+        // dependency chains are scheduled early on in the build process and the
+        // leafs higher in the tree can fill in the cracks later.
+        //
+        // TODO: it'd be best here to throw in a heuristic of crate size as
+        //       well. For example how long did this crate historically take to
+        //       compile? How large is its source code? etc.
+        let next = self.dep_map.iter()
+            .filter(|&(_, &(ref deps, _))| deps.is_empty())
+            .map(|(key, _)| key.clone())
+            .max_by_key(|k| self.depth[k]);
+        let key = match next {
             Some(key) => key,
             None => return None
         };
@@ -142,3 +192,34 @@ impl<K: Hash + Eq + Clone, V> DependencyQueue<K, V> {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::{DependencyQueue, Freshness};
+
+    #[test]
+    fn deep_first() {
+        let mut q = DependencyQueue::new();
+
+        q.queue(Freshness::Fresh, 1, (), &[]);
+        q.queue(Freshness::Fresh, 2, (), &[1]);
+        q.queue(Freshness::Fresh, 3, (), &[]);
+        q.queue(Freshness::Fresh, 4, (), &[2, 3]);
+        q.queue(Freshness::Fresh, 5, (), &[4, 3]);
+        q.queue_finished();
+
+        assert_eq!(q.dequeue(), Some((Freshness::Fresh, 1, ())));
+        assert_eq!(q.dequeue(), Some((Freshness::Fresh, 3, ())));
+        assert_eq!(q.dequeue(), None);
+        q.finish(&3, Freshness::Fresh);
+        assert_eq!(q.dequeue(), None);
+        q.finish(&1, Freshness::Fresh);
+        assert_eq!(q.dequeue(), Some((Freshness::Fresh, 2, ())));
+        assert_eq!(q.dequeue(), None);
+        q.finish(&2, Freshness::Fresh);
+        assert_eq!(q.dequeue(), Some((Freshness::Fresh, 4, ())));
+        assert_eq!(q.dequeue(), None);
+        q.finish(&4, Freshness::Fresh);
+        assert_eq!(q.dequeue(), Some((Freshness::Fresh, 5, ())));
+    }
+}