Add reduced-collection-frequency for niche use case

crossbeam-deque/Cargo.toml

@@ -23,6 +23,8 @@ default = ["std"]
 # NOTE: Disabling `std` feature is not supported yet.
 std = ["crossbeam-epoch/std", "crossbeam-utils/std"]
 
+epoch-reduced-collection-frequency = ["crossbeam-epoch/reduced-collection-frequency"]
+
 [dependencies]
 cfg-if = "1"
 

crossbeam-epoch/Cargo.toml

@@ -42,6 +42,14 @@ nightly = ["crossbeam-utils/nightly"]
 # patch versions of crossbeam may make breaking changes to them at any time.
 loom = ["loom-crate", "crossbeam-utils/loom"]
 
+# Reduce collection frequency
+#
+# NOTE: This feature is to avoid the O(N) overhead, which only manifests under
+# the situation where excessive number of threads (= N) are all accessing to
+# the shared Global inside a single process space. This feature might introduce
+# a potential slow memory reclamation as a trade-off.
+reduced-collection-frequency = []
+
 [build-dependencies]
 autocfg = "1"
 

crossbeam-epoch/src/internal.rs

@@ -387,8 +387,15 @@ fn local_size() {
 impl Local {
     /// Number of pinnings after which a participant will execute some deferred functions from the
     /// global queue.
+    #[cfg(not(feature = "reduced-collection-frequency"))]
     const PINNINGS_BETWEEN_COLLECT: usize = 128;
 
+    #[cfg(feature = "reduced-collection-frequency")]
+    // ~10% collection overhead is observed with the normal 128 frequency with rayon under
+    // 1000-2000 threads including inactive ones. Reduce by ~100x factor to attain
+    // <1% overhead by large margin.
+    const PINNINGS_BETWEEN_COLLECT: usize = 128 * 128;
+
     /// Registers a new `Local` in the provided `Global`.
     pub(crate) fn register(collector: &Collector) -> LocalHandle {
         unsafe {