epoch: Skip try_advance in collect() when the queue is empty

crossbeam-epoch/src/internal.rs

@@ -206,6 +206,15 @@ impl Global {
     /// `collect()` is not called.
     #[cold]
     pub(crate) fn collect(&self, guard: &Guard) {
+        // Advancing the epoch is only useful to make queued bags expirable, so if the queue is
+        // empty there is nothing to collect and the expensive `try_advance` (a `SeqCst` fence plus
+        // a traversal of every registered participant) is pure overhead. Skipping it only delays
+        // epoch advancement, never advances it early, so reclamation stays sound: any thread that
+        // pushed a bag will itself advance the epoch on a later `collect`.
+        if self.queue.is_empty(guard) {
+            return;
+        }
+
         let global_epoch = self.try_advance(guard);
 
         let steps = if cfg!(crossbeam_sanitize) {

crossbeam-epoch/src/sync/queue.rs

@@ -200,6 +200,13 @@ impl<T> Queue<T> {
             }
         }
     }
+
+    /// Returns `true` if the queue is observed to be empty.
+    pub(crate) fn is_empty(&self, guard: &Guard) -> bool {
+        let head = self.head.load(Acquire, guard);
+        let h = unsafe { head.deref() };
+        h.next.load(Acquire, guard).is_null()
+    }
 }
 
 impl<T> Drop for Queue<T> {